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Musings of an Energy Nerd

Designing a Good Ventilation System

Ventilating is easy — it’s ventilating right that’s hard

Image 1 of 2
The gold standard for residential ventilation. The most effective ventilation systems include a heat-recovery ventilator (HRV) or energy-recovery ventilator (ERV) — similar appliances that transfer heat (but not air) between a ventilation system's exhaust and supply air streams.
Image Credit: Renewaire

UPDATED on June 25, 2018

Most green builders include some type of mechanical ventilation system in every home they build. That’s good. Since green buildings usually have very low levels of air leakage, mechanical ventilation is usually essential.

Unfortunately, several research studies have shown that a high number of mechanical ventilation systems are poorly designed or installed. Among the common problems:

  • Ventilation fans with low airflow because of ducts that are undersized, crimped, convoluted, or excessively long.
  • Ventilation systems that ventilate at too high a rate, or for too many hours per day, resulting in a severe energy penalty.
  • Ventilation systems that waste energy because they depend on inappropriate fans (for example, 800-watt furnace blowers).

It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned.

If you’re unfamiliar with residential ventilation systems, it’s a good idea to review the ventilation information in the GreenBuildingAdvisor encyclopedia.

The ASHRAE standard

ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.

The residential ventilation requirements in the 2018 International Residential Code (IRC) differ from the requirements of the ASHRAE 62.2 standard, however. According to the 2018 IRC, the minimum ventilation rate is 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. (For more information on code requirements for ventilation, see “An Update on the Residential Ventilation Debate.”)

Systems complying with ASHRAE 62.2 have ventilation rates that are relatively low; for example, a 2,000-square-foot house with three occupants requires 83 cfm of mechanical ventilation. That’s about as much airflow as is provided by a typical bath exhaust fan. (Of course, systems complying with the minimum requirements of the 2018 IRC have even lower ventilation rates.)

Since ventilation airflows are typically quite low, ventilation ductwork needs to be impeccably sealed. If ventilation ductwork is leaky, fresh air won’t reach its intended destination.

Prominent building scientists are now debating the merits of the ASHRAE 62.2 ventilation rate. Max Sherman, former chairman of the ASHRAE 62.2 committee, defends the existing ASHRAE formula. On the other hand, Joseph Lstiburek, the well-known building scientist and gadfly, argues that the existing ASHRAE ventilation rate is too high, resulting in unnecessarily high energy costs — especially in hot humid climates, where the introduction of high volumes of outdoor air increases the need for cooling and dehumidification.

Lstiburek and Armin Rudd, a fellow engineer at the Building Science Corporation, advise designers of Building America houses to ventilate at a lower rate. “These [Building America] homes have roughly 50 to 60 percent of the ventilation rate required by ASHRAE standard 62.2,” Rudd has written. “The lack of complaints by occupants indicates that the systems are working to provide indoor air quality acceptable to the occupants.”

The “great rate debate” is far from settled; stay tuned. (For more information on this topic, see two articles: Ventilation Rates and Human Health and How Much Fresh Air Does Your Home Need? On August 7, 2013, Joseph Lstiburek released a new proposed ventilation standard, )

Do we really need mechanical ventilation?

As more and more local building codes include ventilation requirements, fewer builders are able to get away with building new homes without mechanical ventilation. However, a few die-hard holdouts defend homes without mechanical ventilation.

One reason why homes without mechanical ventilation systems work better than expected is that many common household appliances act just like exhaust-only ventilation systems. Such appliances include:

  • Power-vented water heaters (50 cfm),
  • Clothes dryers (100 to 225 cfm),
  • Central vacuum cleaners (100 to 200 cfm), and
  • Wood stoves (30 to 50 cfm).

When these appliances are operating, fresh outdoor air enters a house through random cracks to replace the air that is exhausted.

However, homes without ventilation systems are homes of the past. The building science community has reached a consensus: build tight and ventilate right.

What are my choices?

After two decades of experimentation, builders have narrowed ventilation options down to four main options:

  • The simplest system is an exhaust-only ventilation system based on one or more bath exhaust fans.
  • For better fresh air distribution, choose a central-fan-integrated supply ventilation system.
  • For the lowest operating cost, choose a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) connected to a dedicated duct system.
  • If you don’t relish the thought of installing complicated ventilation ductwork, consider installing one or more pairs of innovative from Europe.

Can I install a supply-only ventilation system in a cold climate?

Some builders worry that a supply-only ventilation system (for example, central-fan-integrated supply ventilation) won’t work in a cold climate, because the ventilation fan will drive interior air into building cavities where moisture can condense.

This worry is needless. As energy expert Bruce Harley explains, “The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effect. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes. … In cold climates, I believe that distributed, supply-only ventilation such as that supplied by a ducted distribution system controlled by an , or other ducted low-flow supply ventilation, is vastly preferable to single or multi-port exhaust-only systems, except in extremely tight homes (in which case balanced supply and exhaust ventilation is the best choice).”

What’s wrong with exhaust-only systems?

As Harley’s comments make clear, many energy experts (including Lstiburek) disparage exhaust-only ventilation systems. The main argument against exhaust-only ventilation systems — for example, a Panasonic bath exhaust fan controlled by a timer — is that they don’t provide adequate distribution of fresh air. As a result, some rooms have plenty of fresh air while other rooms remain stuffy. (For more information on this issue, see Ensuring Fresh Air in Bedrooms.)

According to some ventilation experts, ASHRAE 62.2 — which currently lacks any provision requiring fresh-air distribution — should be revised to include a distribution requirement. Armin Rudd has written, “I think distribution of ventilation air is an important issue. Bringing in ventilation air and hoping that it will provide adequate indoor air quality throughout the whole house is just a hope and a prayer.”

Research shows, however, that in some homes — especially small homes with an open floor plan — exhaust-only ventilation systems work well. If the exhaust fan is well chosen — my own favorite is the Panasonic Whisper Green fan, which uses only 11.3 watts to move 80 cfm — exhaust-only ventilation systems have very low installation and operating costs.

If you choose this type of ventilation system, it’s important to remember to undercut the bathroom door.

Do I need passive air inlets?

Most homes with exhaust-only ventilation systems don’t require any passive fresh air inlets in the walls. Unless the house is unusually airtight, fresh air will find its way into the home through random cracks. (For more information on this issue, see Passive Air Inlets Usually Don’t Work.)

A 2000 Vermont study (“A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont”) by Andy Shapiro, David Cawley, and Jeremy King, investigated whether passive fresh air inlets make any sense. The researchers studied 43 new homes (22 of which had passive fresh air vents) with exhaust-only ventilation systems. They wrote, “When the EOV [exhaust-only ventilation] fan was operating, 35% of the vents were exhausting inside air, 48% were supplying outside air, and 17% of the vents were not moving air.” The explanation? “The pressures induced by fans in these [studied homes] … were low relative to pressures induced on a house by natural forces, including wind and temperature-driven stack effect.”

Note that there is an exception to this guideline: if your house approaches Passivhaus levels of airtightness (aiming for 1 air change per hour at 50 Pascals or less), an exhaust-only ventilation system may be starved for makeup air. Most Passivhaus homes have a balanced ventilation system (an HRV or an ERV). Builders of very tight homes who prefer to install an exhaust-only ventilation system should consider the installation of one or two passive air inlets.

There’s an easy way to check whether your exhaust fan is starved for makeup air: simply measure the exhaust air flow. If you are aiming for 50 cfm of exhaust air flow, and that’s what you’re getting, then everything is fine. If 50 cfm of air is leaving your house, that means that 50 cfm of outdoor air is simultaneously entering your house.

Central-fan-integrated supply ventilation

For years, the engineers at the Building Science Corporation have been singing the praises of central-fan-integrated supply ventilation systems. These systems can only be used in homes with forced-air heating or cooling systems. The systems include three important components:

  • A duct that introduces outdoor air to the furnace’s return-air plenum;
  • A motorized damper in the fresh air duct;
  • An to monitor the run-time of the furnace blower and to control the motorized damper.

The AirCycler control (also known as a FanCycler) prevents both underventilation and overventilation. When the AirCycler notices that the furnace fan hasn’t operated for a long time, the control turns on the fan to prevent underventilation. When the control notices that the fan has been operating continuously for a long time, the control closes the motorized damper to prevent overventilation.

During the swing seasons — spring and fall — the furnace blower will need to operate occasionally for ventilation purposes, even when there is no call for heat or cooling. In most climates, about 15% of the annual blower run time for such systems will be devoted to ventilation only. If the system is properly commissioned, the furnace will supply a 7% outside air fraction during ventilation mode.

The big downside to central-fan-integrated supply ventilation is that the installer needs to understand how to design and commission the system. HVAC contractors capable of this task are rare. Unless the designer of a central-fan-integrated ventilation system takes great care when specifying the furnace and programming blower operation, such a system can have unreasonably high operating costs.

A well-designed central-fan-integrated supply ventilation system needs a furnace with an energy-efficient ECM blower. Such furnaces cost between $1,000 and $1,500 more than conventional furnaces. If you end up using a furnace with a conventional blower motor — that is, one that draws 700 to 800 watts — the ventilation system will incur a big energy penalty. (For purposes of comparison, a Panasonic exhaust fan draws 11.3 watts, and most HRVs draw 100 watts or less).

Duct systems and fans designed for heating and cooling are not optimized for ventilation. While ventilation airflow is typically in the range of 50 to 100 cfm, furnace fans move as much as 1,200 to 1,400 cfm. One study (Robb Aldrich, Chicago, 2005) found that a poorly designed central-fan-integrated supply ventilation system in a house with an 800-watt furnace fan used 347 kWh of electricity for ventilation during a swing-season month. During the same month, an identical home with an exhaust-only ventilation system used only 6% as much electricity for ventilation. Although the researchers were somewhat worried that the exhaust-only ventilation system might be ineffective, the data were reassuring: all of the rooms had very acceptable CO2 readings.

Will cold outdoor air damage my furnace?

Some builders worry that central-fan-integrated supply ventilation systems won’t work in a cold climate, where cold outdoor air might damage the furnace. According to Armin Rudd, such concerns are baseless — as long as the ventilation system is well designed.

Assuming a high outdoor air fraction (15%) and a low outdoor temperature (-30°F), a furnace equipped with a supply-only ventilation system will experience mixed return-air temperatures no colder than 55°F, as long as the thermostat is set to 70°F. Even in Chicago, such systems work well.

Do I really need the AirCycler and motorized damper?

To reduce costs, some builders install the lazy man’s version of a central-fan-integrated supply ventilation system — one that includes a passive fresh air duct to the return-air plenum, but without a motorized damper or AirCycler control.

What’s wrong with this approach?

  • During the swing seasons, when the furnace fan isn’t operating, the house won’t get enough fresh outdoor air, and homeowners may complain of stuffiness.
  • During the rest of the year, when the furnace fan is operating regularly, the house will be overventilated, resulting an a severe energy penalty. During the winter, all that unnecessary cold air will need to be heated; during the summer, all that unnecessary hot air will need to be cooled and dehumidified.

An HRV with dedicated ventilation ductwork

The best ventilation performance and lowest operating cost comes from an HRV or ERV with dedicated ventilation ductwork. Such a “gold standard” system should be designed to pull stale air from bathrooms and laundry rooms, while introducing fresh air to the living room and bedrooms. [Author’s postscript: After this article was written, a new type of energy-efficient balanced ventilation system, the CERV, became available in North America. For more information on the CERV, see A Balanced Ventilation System With a Built-In Heat Pump.]

Although HRVs and ERVs save energy compared to exhaust-only or supply-only ventilation systems, they are expensive to install. The high cost of these systems raises questions about their cost-effectiveness, especially in mild climates. To learn more about this issue, see Are HRVs Cost-Effective?

For ventilation purposes, either an HRV or an ERV can work well in any climate. The presumed advantage of ERVs over HRVs in hot, humid climates is not based on research or field data. As Max Sherman has written, “Almost all hot, humid climates have hours when it is dryer outside than inside, and then ERVs actually make the [indoor] moisture problem worse. The net effect this that ERVs are about a wash [compared to HRVs] for humidity control in those climates.” (For more information on this topic, see “HRV or ERV?”)

Lunos fans

The Lunos fan is a new type of ventilation fan from Germany. Installed in pairs, the wall-mounted ventilation fans automatically alternate between exhaust mode and supply mode. Because each fan includes a ceramic core, they are able to recover heat from the exhaust air stream.

These fans are particularly useful for retrofit applications, or for any situation where the installation of ductwork would be awkward. For more information, see European Products for Building Tight Homes.

To commission a ventilation system, you need to measure airflow

Anyone who commissions a ventilation system needs to learn how to measure airflow. Manufacturers offer an array of accurate (and expensive) instruments to measure airflow, including $2,000 flow hoods. Builders who need to troubleshoot problems may be interested in several lower-cost methods of measuring airflow, including the use of a home-made flow hood, a method requiring a cardboard box and an old credit card, the garbage-bag method, and a method using a laundry basket or wastebasket.

For more information on these test methods, see these two articles:

Last week’s blog: “Farewell to the Chimney?”

115 Comments

  1. User avater
    David Bearg, PE, CIH | | #1

    Pathway for Replacement Air
    Martin:

    I was talking with Victor, your old college roommate at a contra dance last night, and he sends his regards.

    Any experience with using a duct through a basement wall that terminates in a back as a device for allowing replacement air to be pulled into the house by the operation of the exhaust fans?

    Also, I invite you to visit my site and download and read my article on "Achieving and Maintaining Healthy Lakesidecas"

    David

  2. User avater
    David Bearg, PE, CIH | | #2

    Pathway for Replacement Air
    Oops, typo:

    The outdoor air duct terminates in a BUCKET. The bucket fills with cold air and airs will only enter to replace air that has left the house elsewhere.

    David (again)

  3. User avater GBA Editor
    Martin Holladay | | #3

    Makeup air
    David,
    Concerning a passive duct that introduces exterior air into a basement: such a duct is a crude source of makeup air, whether or not it terminates in a bucket. As you point out, the duct will be a source of makeup air whenever the house is depressurized. Depressurization can occur for a variety of reasons, including operation of combustion equipment, operation of exhaust fans, or the stack effect.

    The trouble with such a passive duct is that it is basically an uncontrolled hole in the house -- never a good idea. All winter long, the stack effect causes the upper half of the typical home to be pressurized. Air at the top of the house is continually escaping through small cracks in the ceiling and upper wall area. If there's a big hole in the basement -- for example, a passive makeup air duct -- the stack effect is greatly increased, and the house will lose air continuously. Of course, a tight ceiling will help. But even a well-sealed house has leaks in the ceiling.

    Say hi to Victor when you see him again, David.

  4. Riversong | | #4

    Make up air inlets
    You disparage exhaust-only ventilation systems as not providing good distribution of fresh air and then discourage the passive make-up air inlets that would allow positive distribution in the spaces where it's most needed - bedrooms and living rooms.

    While it's true that putting deliberate "holes" in the thermal envelope can cause havoc in a poorly-sealed house that experiences stack losses and wind pressurization; in a house that's built tightly to today's air barrier specifications, passive make-up air inlets DO work because a small bath exhaust fan will easily overcome the minimal stack effect pressure.

    And an exhaust-only ventilation system is the only option which will maintain a constant negative pressure in the conditioned space, preventing air exfiltration into the thermal envelope. "Balanced" power ventilation systems can pressurize a house because the cold winter air expands when warmed which results in more cfm input than output.

  5. Riversong | | #5

    neutral pressure plane
    You say: As energy expert Bruce Harley explains, "...The relatively small airflow of most supply-only ventilation systems...will have little effect on this situation other than to shift the neutral pressure plane down slightly..."

    Running an exhaust fan (i.e. increasing negative pressure or decreasing positive pressure) will RAISE the neutral pressure plane. If the entire conditioned space is under negative pressure, the neutral pressure plane will be at the upstairs ceiling (an ideal situation in a cold climate).

  6. User avater GBA Editor
    Martin Holladay | | #6

    Passive air inlets
    Robert,
    I hope my summary of ventilation options is not seen as disparaging exhaust-only ventilation systems. For small houses with an open floor plan, they make a lot of sense. They are affordable and energy efficient.

    However, I disagree with two of your points:

    1. It is simply untrue that "a small bath exhaust fan will easily overcome the minimal stack effect pressure." The research paper I cited -- “A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont” -- set out to investigate precisely this question. According to measurements made by the researchers, “The pressures induced by fans in these [studied homes] … were low relative to pressures induced on a house by natural forces, including wind and temperature-driven stack effect.” That's why they discovered that, even when the exhaust fan was operating, 35% of the passive air vents were exhausting inside air rather than admitting fresh air, and 17% of the passive air vents were not moving air.

    2. It is also (sadly) untrue that "passive make-up air inlets ... allow positive distribution in the spaces where it's most needed - bedrooms and living rooms." I wish it were true, but it isn't. Good fresh air distribution requires the use of either a central-fan-integrated supply ventilation system or an HRV with dedicated ductwork. Even if you pepper your walls with holes (passive air inlets), there's no guarantee than any fresh air will enter the holes. For example, if your bedrooms are on the second floor, there's an excellent chance that the stack effect will undermine the operation of the passive air inlets, allowing conditioned air to escape or preventing any air movement through them at all.

    However, these distribution problems don't always rise to a level of concern. Homes with well-designed exhaust-only ventilation systems often have happy occupants without any complaints. The systems do provide fresh air, even if it isn't perfectly distributed, and in many cases these systems are perfectly adequate.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Exhaust systems, supply systems
    Robert,
    You object to my quote from Bruce Harley concerning supply-only ventilation systems. Your counter-example, however, concerns use of an exhaust fan. But Harley is talking about systems with a supply air fan, not an exhaust fan.

  8. Riversong | | #8

    Flawed Study
    My mistake on misreading Harley's context.

    But your rebuttals of my statements as "simply untrue" is based on a single, highly flawed study. Even a cursory reading of the study (and I just read every word) reveals that the only legitimate conclusion would be that EOV systems don't function well in not very tight, poorly designed homes with shamefully inadequate construction detailing.

    45% of the builders failed to program the fan timer, one had a blocked duct and the average measured fan flow was only 67% of factory rating. 70% had a gap between fan housing and ceiling drywall, with one having 5 square inches! Only half the homes even used passive air inlets, with an average of only 3.7 per home - even though the homes averaged almost 2200 SF and 3.3 bedrooms. 19 systems used flex duct rather than rigid, smooth-wall. Only 43% had a discernible pressure difference at the inlet with the fan running, and the average depressurization was only 1 Pascal.

    In the last superinsulated house I built (the only one for which I have blower door data), the ACH50 was 2.13 with the air inlets taped and 3.06 with them open. The baseline house pressure (24° outside temp, no wind) was -2.2 Pa. With one bath fan running it was -4 Pa and with both fans -8.6 Pa.

    The Panasonic bath fans had measured air flows of 86% and 92% of factory rating. The seven Airlet 100s were passing an average of 7.13 cfm with both bath fans running (though this was with the weatherstripped mud/laundry room door open which allowed almost 11 cfm to enter through the dryer make-up air duct, and there was 18 cfm leaking in through the woodstove combustion air inlet because the woodstove hadn't been installed).

    While it's true that, under these conditions the air inlets weren't providing all the make-up airflow for the fan exhaust, and some was leaking through the minimal effective leakage area of the air-tight shell, both the fans and inlets were working as designed - contrary to the study you reference.

    Conclusion: a programmed exhaust-only ventilation system with passive make-up inlets in a very tight house will function as intended and can meet the ASHRAE IAQ standards at minimal cost with minimal ducting.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Where we agree
    Robert,
    Thanks for posting your views. Although you disparage the study conducted by Andy Shapiro, David Cawley, and Jeremy King as "highly flawed," and imply that my reliance on their data is misguided, I think the researchers are due a little more respect than you grant them.

    Because this was a field study, the researchers measured the actual performance of exhaust ventilation systems in existing new homes. They were not measuring the performance of installations in a lab. As you point out, many of the systems they looked at were poorly designed or poorly installed. Their data are useful, though. One of the reasons I wrote this blog — entirely consistent with their findings — was expressed toward the top of my essay: "It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned."

    Although you point out that "only half the homes even had passive air inlets," the researchers were fully aware of that fact. Their conclusions concerning the performance of passive air inlets were based only on data from the 22 homes equipped with the devices.

    The researchers concluded that random air leaks are a perfectly adequate source of makeup air for exhaust-only ventilation systems. It would appear that the builders who omitted passive air inlets made the right decision, so I'm not sure why you emphasize that "only" half the homes had passive air inlets.

    The three researchers took pains to measure any despressurization caused by exhaust fans in these houses. They wrote, "It should be noted that measuring these low pressures was difficult, because the wind could and often did overpower the small pressure induced by the fan. Values would fluctuate widely when the wind was blowing. In only 43% of the cases was it clear that running the fan induced a discernible pressure difference at the passive air vent."

    I'd like to sum up by emphasizing our points of agreement:

    1. Many builders are currently doing a sloppy job when installing exhaust fans. It's important to get the details right. That means that ducts should not be convoluted, should have as few elbows as possible, and should have sealed joints. Moreover, builders need to seal the gap between the fan housing and the drywall.

    2. All of us need to redouble our air sealing efforts. Tight homes perform better than leaky homes. Those who are achieving good results -- including you, Robert -- deserve accolades.

    3. Exhaust-only ventilation systems can work well. Most occupants of homes with exhaust-only ventilation systems have no complaints.

    Our main disagreement, it appears, concerns the usefulness of passive air inlets. I still feel that the study I cited provides ample evidence that random air leaks are a perfectly adequate source of makeup air for exhaust-only ventilation systems. Robert, it appears that you prefer to install passive air inlets. This disagreement is minor in light of the points on which we can agree.

  10. John Brooks | | #10

    I agree with Riversong
    Martin,
    I think that we should be very careful about drawing conclusions from data taken in average new homes and even in Energy Star level homes.
    All new homes should be built airtight.PERIOD
    Relying on make-up air from an unknown source is a bad idea.
    There should be pressure balancing between all spaces between intake and exhaust.
    HRV is best .. but I think that Robert's Plan is sound and would/will work in a properly built home.

  11. User avater GBA Editor
    Martin Holladay | | #11

    Still looking for agreement
    John,
    I agree that an HRV is best.

    Unfortunately, no one has yet been able to build an airtight house. There are a few exceptions, perhaps, including submarines and the Space Shuttle; but in general we measure airtightness with a blower door because all homes leak.

    The homes in the study under discussion had air leakage rates that covered a wide range, as is typical for a group of new homes. The best home fell well short of the Passivhaus standard, of course; it measured 2.24 ac/h @ 50 Pa -- about the same as the superinsulated house that Riversong built.

    Depending on "random leaks" for makeup air is not as dangerous as some building experts maintain, although scare stories about makeup air from garages and crawl spaces are commonly bandied about at building science conferences. Obviously, it's important to do a very good job air sealing a common wall between an attached garage and living space.

    There is even some evidence that air that enters a house through wall leaks can be cleaner than outdoor air, because fiberglass insulation can act as an air filter.

    Here's the bottom line: installing a bunch of "fresh air vents" in your walls is all fine and good, but there's no guarantee that outdoor air will follow your instructions and use your designated holes. Those arrows in the ads from air inlet manufacturers aren't "smart arrows." They're more like the "serving suggestion" shown on a box of corn flakes. In most cases, the strawberries don't come with the box.

  12. John Brooks | | #12

    We can do it
    We can build better than 2.24 ach50
    How well did the air inlets work in the The best house?
    I think that...
    If you are going to use exhaust ventilation then providing the air inlets would be the wiser choice.
    And that goes without saying that you must measure the flow..
    If the flow is not there... the problem is not the the inclusion of air inlets. The air inlets would increase your odds of achieving a good outcome.

  13. User avater
    Carl Seville | | #13

    Great stuff
    Martin
    Thanks for the solid info on this subject. Ventilation is definitely a big issue with tight homes and I have found a wide range of professional opinions on how much and how to accomplish it. Considering that my experience and most of my work is in mixed humid climates, what do you think about using a dehumidifier with a fan cycler function running through the central HVAC ducts for fresh air ventilation? Where it isn't frigid the winter energy penalty would be minor, and the dehumidifier should handle the excess moisture brought in. This should also solve the fan energy issue as long as the blower used is efficient enough.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Fan Cycler and dehumidifier
    Carl,
    Ventilating with a passive air duct connected to your central HVAC ducts is a fine way to ventilate a house, as long as you remember:

    1. Only use a furnace or air handler with an ECM blower, since conventional blower motors are energy hogs.

    2. The fresh air duct must include a motorized damper controlled by the FanCycler unit.

    However, I'm not a proponent of whole-house dehumidification using central HVAC ducts. I base my belief on a great study performed in Houston by Armin Rudd. (Rudd presented the study on October 10, 2002 at the EEBA conference in Phoenix, Arizona. A report of his findings appeared in the January 2003 issue of Energy Design Update.)

    In a hot, humid climate, Rudd compared six dehumidification strategies in energy-efficient new homes. These included the use of an Ultra-Aire dehumidifier ($1,250), a Venmar ERV ($1,450), a Carrier 2-stage air conditioner ($1,550 incremental cost over a regular air conditioner). There were other tested strategies as well.

    Surprisingly, the most effective dehumidification strategy was the cheapest and simplest: installing a stand-alone Whirlpool dehumidifier in an interior closet. Total installed cost: $500. The fancy equipment cost more to install, used more energy, and was less effective than the simple portable dehumidifier.

  15. John Brooks | | #15

    Agree on the stand-alone dehumidifier
    Martin,
    In a Hot/Mixed Humid climate...
    I have the forced air type system with air controller and motorized damper and ECM motor as you described.
    Works very well during the summer and part of the shoulder season.
    I also have a stand-alone dehumidifier ($250 Lowes) for those special occasions.... a few times in winter when Air Cycler was not keeping the RH down low enough...and a few times in the shoulder seasons when the RH was high outside and there was no call for the AC.
    Next time I will build a little bit tighter add more wall insulation and go for a dedicated HRV.. (also Cool Recovery Ventilator)... and still use a stand-alone dehumidifier.

  16. John Brooks | | #16

    Faster air changes... Flushing
    I think that our fresh air systems should also be "capable" of more airchanges than mandated by 62.2
    When the Weather outside is better than inside.. It would be nice to be able to rapidly flush our homes with "better air"

    I know what your going to say... just open the windows.
    I think that we should consider minimizing our operable windows and use a higher percentage of picture windows...
    According to Thorsten Chlupp it is much easier to insulate over the frame of a picture window and really boost performance.
    Of course we still need fire escape windows or "little doors" and a way to ventilate if the power goes down.
    Our current home is very comfortable..we almost never open the windows.. If we really want to feel like we are outside...WE GO OUTSIDE

  17. User avater GBA Editor
    Martin Holladay | | #17

    Whole-house fan
    John,
    The product you are describing exists; it's called a whole-house fan. It's used to flush the air out of the house whenever the outdoor air is more pleasant than the indoor air. It is an exhaust fan designed to be installed in the upstairs ceiling; when used — usually at night, during the summer, to cool the house — downstairs windows are left open.

    For decades, my grandfather, a former president of ASHRAE, used a whole-house fan originally installed in the 1930s or 1940. He lived in southern California, and never installed air conditioning in his house. "There's no need for air conditioning. The whole-house fan works better," he used to say.

    The best whole-house fan on the market is made by Tamarack Technologies:

    A whole-house fan is fundamentally different from a fan used for residential mechanical ventilation because it moves a lot more air. While a mechanical ventilation fan needs to move 40 to 80 cfm, a whole-house fan generally moves 1,000 cfm to 2,000 cfm or more.

  18. User avater
    Carl Seville | | #18

    Need More Info
    Martin

    thanks for the info on the dehumidification, but it leaves me with some more questions.
    1 - Why the motorized damper instead of a barometric one if the house is so tight that it needs fresh air more than the air handler will run? The last few homes I have rated were so tight that they needed the blower running about 2/3 of the time to meet 62.2

    2 - If a stand alone dehumidifier is installed in a closet, would it be appropriate to put supply and return ducts in the same closet to circulate the dry air around the house?

  19. User avater GBA Editor
    Martin Holladay | | #19

    Further details
    Carl,
    1. In almost all cases, a barometric damper in the fresh air duct will result in overventilation on the coldest and hottest days of the year — precisely the time when it is most expensive to condition outdoor ventilation air. If properly sized, the furnace will run flat out on the coldest day of the year, and the air conditioner will run flat out on the hottest day of the year. How much ventilation air is being introduced when the HVAC blower runs for 20 or 24 hours a day? That depends -- you'll have to measure it. Armin Rudd likes to set up his systems for a 7% outside air fraction. You need to know the rating of your blower, too. If you have a 1,500 cfm blower and a 7% outside air fraction, you are ventilating at 105 cfm. Depending on the size of your house, that might be just right -- or it might be twice as much outside air as you need. Here's the rub, though: a lot of HVAC installers don't measure the air flow through the fresh air duct when they commission the system. (Because they don't really commission the system.) They just hook up a 6-inch duct and they're done. In that case, you might end up with a 15% or 20% outside air fraction, and you are really overventilating. The outside air fraction needs to be measured, and a damper (not the motorized damper, just a regular damper) needs to be adjusted to get the outside air fraction right. Then, you still need a motorized damper so that the FanCycler control can shut the fresh air duct when the blower has been running for hours at a stretch. Remember, Rudd advocates ventilating at LESS than the ASHRAE 62.2 rate. I'm not saying you should do that, but a lot of people do, especially in hot, humid climates.

    2. Here's how Armin Rudd installed the the stand-alone dehumidifiers that performed so well in Houston: "A stand-alone dehumidifier was
    installed in a hall closet equipped with a louvered door. This is an inexpensive, tried-and-true system, using a widely available $225 off-the-shelf dehumidifier. Although the closet was not far from the house’s main return-air grille, the dehumidifier was not located directly in the return airstream. The dehumidifier’s drain was tied to the house’s drain system."

  20. User avater
    Carl Seville | | #20

    62.2 - Can the experts make up their minds?
    Thanks for the info. I have read Armin's opinion that 62.2 is too much, and have had discussions with Terry Brennan who says it is the absolute minimum. Who the hell are we supposed to listen to? I respect both of them very much, but it is getting very confusing out there.

  21. John Brooks | | #21

    HRV makes more and more sense
    Once we have crossed the hurdle of building airtight (0.6 ACH50) and attack the weak points in the thermal envelope...The HRV really makes sense.

    We could ventilate at 62.2 and greater and not have to worry about the "air change penalty"

  22. User avater GBA Editor
    Martin Holladay | | #22

    Who do we listen to?
    Carl,
    In response to your question, "Who do we listen to?," here's some advice:

    1. In every house you build, install a mechanical ventilation system capable of providing ventilation that meets ASHRAE 62.2 requirements.

    2. In your own home, experiment with lower ventilation rates by adjusting the programming of your ventilation equipment. If you have no complaints — no condensation on your windows during the winter, no mold, no stuffiness, and no smelly-sock odors — you're okay.

    3. In your clients' homes, remember this: regardless of how you program the ventilation equipment, once the clients move in, they will operate it any way they want. This is a fact — a sometimes worrisome fact, but a fact nevertheless. They may end up ventilating at a lower rate than 62.2 — perhaps by listening to your advice, or perhaps by experimentation. Or they may run the ventilation equipment for 24/7. You can advise, but you can't compel.

    One worrisome element to this issue is that some homes can be damaged by the owners' decision not to ventilate. That's one reason to choose wall and ceiling assemblies that are durable and not prone to condensation. You don't want to depend on your clients' decision to ventilate to maintain the structural integrity of your buildings.

  23. User avater GBA Editor
    Martin Holladay | | #23

    HRV energy penalties
    John,
    Every type of mechanical ventilation system, including an HRV, comes with an energy penalty. HRVs require electricity to run; that's an energy penalty. HRV efficiency is always less than 100%; it may be 58%, or it may be 89%, but it's not 100%. Therefore HRVs have a second energy penalty in addition to the electricity they consume: they require the furnace and air conditioner to run more than they would otherwise, to condition the ventilation air.

    Two research studies looked at the energy penalty arising from the use of HRVs. With energy costs calculated in 1998, a Lawrence Berkeley study calculated that an HRV in a typical home costs $168 to $193 per year to operate, depending on climate. In 2001, NAHB researchers calculated the cost to operate an HRV at $219 to $266 per year.

    The systems studied by these researchers were typical systems, but were not optimized for minimum energy use. If designed and installed by a smart cookie, an HRV system might have a lower energy penalty than the ones studied by these researchers.

  24. John Brooks | | #24

    Research Study?
    Of course the HRV must be integrated into the design..
    I suspect that the systems studied were tacked onto the houses much like the way we typically install our ac systems.
    Look at the HRV in Alex's Super Insulated remodel

    (Scroll down about halfway)

    I caution again about drawing conclusions from studies conducted in typical homes.
    It works in Germany because it is not an afterthought and the houses are designed as systems.

    And we (USA) need better HRV's like the Germans..with ECM motors and higher effeciency ratings.

  25. Stephen Bennett | | #25

    Passive Dehumidification
    Hello
    I have a very poor sealed 19th century church I am fixing up and living in.
    It is very humid downstairs.
    I need a passive system to move warm dry air downward and cool wet air upward.
    I am thinking of a simple PVC snorkel with fan but probably need something a bit more developed or efficient. Is there any kit out there or plans to make solve this problem.?
    Thks
    Stephen

  26. User avater GBA Editor
    Martin Holladay | | #26

    What's the moisture source?
    Stephen,
    To answer your question properly, we need more information.

    1. The main unanswered question is, What is your moisture source? For example, is it ground moisture entering through a stone floor without a vapor barrier between the floor and the soil below? Or is there another moisture source?

    2. What is your climate? Where is this church?

    3. During what seasons of the year do you notice this humidity problem?

    4. Is the interior of the church conditioned? In other words, is there a heating or air-conditioning system?

    5. What makes you think that the air near the ceiling of your church has a lower humidity level than the air near the floor?

  27. Stephen Bennett | | #27

    Humidity in church
    Martin
    The church has two levels
    the lower lever is half underground the upper floor is above ground
    The warm air from outside comes into the cool underground level and water condensates on the wall and floor which are concrete
    I have heat pumps as air conditioners but they dont seem to be able to handle all the moisture
    I do not have the money to bu an expensive Compressor type humidifier or the money to pay for the electricty to run it
    Each level in the church is 3000 feet
    I am searching for an inexpensive way to de humidify
    Thanks for the response
    any advice would be appreciated
    Stephen

  28. User avater GBA Editor
    Martin Holladay | | #28

    Your suggested solution won't work
    Your problem is that warm, humid summer air contacts the cold concrete walls and concrete floor of the church basement. Predictably, the moisture from the air condenses on these cold surfaces.

    Introducing more warm, humid air into your basement with a fan will not solve the problem; it may in fact increase it.

    Sometimes there is no avoiding the fact that solving a problem will cost money. You have two choices:

    1. Raise the temperature of the condensing surfaces by covering the concrete floor and concrete walls with a layer of extruded polystyrene foam. The foam will have to be protected, of course; I suggest plywood over the floor foam and moisture-resistant wallboard over the wall foam.

    2. Remove moisture from the indoor air by running a dehumidifier.

  29. Riversong | | #29

    Does 62.2 require too much
    Does 62.2 require too much ventilation or not enough?

    That largely depends on the toxicity of both the indoor and outdoor environments, the toxicity of the lifestyle of the occupants, and the occupancy patterns of the residents.

  30. BobR | | #30

    Low cost ERV???
    Martin,

    Are you aware of any lower cost ERVs that have a decent payback period?

    thanks,
    Bob

  31. User avater GBA Editor
    Martin Holladay | | #31

    Payback period?
    Bob,
    Don't expect HRVs, or any other type of mechanical ventilation system, to provide any "payback."
    Every type of mechanical ventilation system has an energy penalty. In other words, there is no payback.

    That doesn't mean that you shouldn't include mechanical ventilation. You should.

  32. Huck | | #32

    Make-up Air - Damper Placement
    We recently moved into our home (in Louisiana) which is tightly sealed. The central room has a 22' ceiling with a large (48") Isokern Magnum fireplace which tends to backdraft unless a window is left partially open. I was thinking about installing a simple barometric passive damper at the neutral pressure plane of the central room (about 11' off the floor). In theory, when the negative pressure rises above the normal neutral plane (because hot air is exhausting through the chimney), the damper would open and prevent backdrafting. This solution is uncomplicated and low cost, but is it too simplistic?

  33. User avater GBA Editor
    Martin Holladay | | #33

    Barometric damper
    Huck,
    The main reason why your barometric damper idea will not work exactly as you envision is wind. (If I remember correctly, Louisiana has wind.) Even a light breeze on the side of the house with the damper will cause infiltration through the barometric damper.

  34. Stephen Colley | | #34

    Ventilation in Hot Humid Climate
    Regardless of any jurisdictional requirement to ASHRAE 62.2, I have not heard anything with involving permits being withheld for lack of installing ventilation strategies to address 62.2. That being said, I am inferring from the above insightful discussion that it only becomes a potentially serious issue with houses built extremely tight (around 2 [email protected] 50?). With issues being stale air and damage due to excessive moisture somewhere leading to mold/decay, it seems like the prudent response in tight, well-insulated construction in hot, humid climates (and maybe elsewhere) would be to have easy access to a good quality relative humidity gauge and a portable dehumidifier that can be manually activated. To solve any stale air issues, it may simply be a matter of opening doors and windows briefly. With a tight and well-insulated house, occasional introduction of outdoor air should be relatively quickly conditioned once the stale air problem has been eliminated. For troublesome areas, such as bathrooms, laundry rooms, I would think a fan-timer-light switch could minimize air quality issues.

  35. Huck | | #35

    Barometric Damper
    Martin--

    Thanks for the quick reply. The location of the damper would allow me to shield it from direct wind effects because the damper's inlet will be in an attic space above the outside porch (that attic area is well-ventilated). My concern about that location is this: during the summer heat will naturally build up in that space and might push the damper open from behind. This would allow hot, humid air into the living space. Should this be a concern? If not, do you believe that a barometric damper in the location I've described is a viable option?

  36. User avater GBA Editor
    Martin Holladay | | #36

    Exterior air pressure
    Huck,
    If the attic is well ventilated, the attic space will have about the same air pressure as the exterior. When the wind blows, the entire side of the house will be pressurized with respect to the interior. The attic you describe will also be pressurized by wind.

    Go ahead and experiment if you want. But such a damper will basically be a hole in your wall.

  37. Edward Pahler | | #37

    passive outdoor air
    It would be a clean air improvement if the outdoor air entering the air return system of the home was filtered. It is my impression that a system which needs to be explored is one where an intake with a damper and a filter breathes fresh air into a conditioned system and the exhaust air is released through a passive vent using the stack pressure at the top floor of the house. Any comments?

  38. User avater GBA Editor
    Martin Holladay | | #38

    Supply-only system
    Edward,
    You are describing a supply-only ventilation system, which is more typically used in warm climates than cold climates. The passive vent is unnecessary, however, since the low ventilation rates required by ASHRAE 62.2 do not require huge volumes of air. Normal cracks in the building envelope easily handle the pressure-equalization of a typical supply-only ventilation system.

    Those concerned about the need to filter ventilation air can also use an HRV or ERV — that is, a balanced ventilation system.

  39. Interested Onlooker | | #39

    Balanced Ventilation Systems
    " "Balanced" power ventilation systems can pressurize a house because the cold winter air expands when warmed which results in more cfm input than output."

    Would the use of an HRV heat exchanger help here by cooling (and shrinking) the outgoing air so that the cfms were more nearly matched?

  40. User avater GBA Editor
    Martin Holladay | | #40

    On balancing balanced systems
    Interested Onlooker,
    Robert Riversong may wish to respond to your question, which was apparently prompted by his earlier post.

    In the meantime, you may be interested to note:
    1.The installer of a balanced ventilation system (such as a system with an HRV or ERV) should always commission the system after installation. Commissioning includes balancing. If it's necessary to meet some special requirement of the house, it's possible to "balance" the system in a way that slightly pressurizes or slightly depressurizes the house.

    2. Of course, the temperature of the outdoor air changes all the time. But this fact doesn't cause an important enough variation in cfm to matter.

  41. Richard Kaye | | #41

    HRV payback again
    Martin,
    Let me ask Bob's question a little differently, and maybe not receive a dismissive answer this time.
    Given that a tightly built home will need a mechanical ventilation system of some sort, can I save enough energy with an HRV to make up the additional cost over a simple bath-fan exhaust-only system? Intuitively, it just seems crazy to heat the air and then throw away all the added heat eight times or so a day.
    Richard

  42. User avater GBA Editor
    Martin Holladay | | #42

    It depends
    Richard,
    The answer to your question depends on the cost of the HRV system, the cost of the exhaust-only system, and the local electricity rates.

    I don't have up-to-date estimates for ventilation system operation costs at my fingertips, but I wrote an article on the topic for the May 2002 issue of Energy Design Update. The average of two estimates for HRV operating costs (one in an NAHB Research Center study, the other in an LBNL study) was $0.576 per day ($210 per year), while the average of two estimates for exhaust-only ventilation systems was $0.76 per day ($277 per year).

    If an HRV system costs $2,000 to install, and an exhaust-only system costs $500 to install, then the simple payback period for the HRV (based on operating cost savings compared to an exhaust-only system) would be 22 years.

    Of course, all of these numbers include assumptions, and all of the assumptions are open to debate and attack. If you are planning your own house, you can do your own math.

  43. Bob Ellenberg | | #43

    Combination system
    I have read this series several times and ALWAYS end up with lots of questions in my mind so I decided to post since I see this discussion has been active as recently as March.

    Here is my planned system: South Louisiana (hot/humid), 1,100 SF extremely tight heavily insulated house with modest quality windows (will allow some leakage) and about 13% wall area in glass. Sealed envelope, conditioned crawl space. Plan to install central AC with heat strips as heat demand will be low, ducts in the conditioned craw space. Bath fan and kitchen fan for point of pollution exhaust and no supply in the bath room for the central system. I have had excellent past results with whole house type fans in the inbetween seasons. I have used the Tamarack fan referenced above but I also used a simple $110 Broan 470 CFM through the wall unit with an insulated positive closure door near the ceiling in a 1,000 SF unit and it worked great. I plan to also install a stand alone dehumidifier as discussed above but I anticipate the heat it generates will be something of a trade off as I'll have to turn the AC on sooner in the late spring.

    Here are my questions: If needed for clean air filtration, I'll install an Ultimate Air unit. But if we have the bath and kitchen fans am I likely to need it? Will those exhaust fans likely cause depressuration problems without a passive inlet (I know the comments above but except for air around the windows, this should be an exceptionally tight house). If I need (or later add) the Ultimate Air unit, during the times of the year that the central system runs for heating and cooling, it should offer sufficient circulation for the filtered air with the supply side of the EVR tied into the central return and the intake near the kitchen area. During the in between season, if the house were closed up and comfortable, I considered that I could flip the main switch off for the air handler and insert a "blank plate" into the filter slot so that the EVR could circulate using the duct system without running the AH or short circuiting the clean air. Is this likely to work?

    Lastly, if there is long enough periods where the 470 CFM fan works to keep the house comfortable, would it be worth putting a screen on the sealed insulated crawl space door so that the air was being drawn through there before coming up to the living area? It will be sealed and conditioned but the cooling effect of the ground would certainly result in cooler air down there.

    All comments and critiques are appreciated.

  44. User avater GBA Editor
    Martin Holladay | | #44

    Response to Bob
    Bob,
    Q. If we have the bath and kitchen fans, am I likely to need an Ultimate Air unit?

    A. I assume your are talking about an ERV. The answer is no -- no one NEEDS an ERV. You install one because you want one, not because you need one.

    Q. Will those exhaust fans likely cause depressurization problems without a passive inlet?

    A: No.

    Q. If I later add the Ultimate Air unit, during the times of the year that the central system runs for heating and cooling, it should offer sufficient circulation for the filtered air with the supply side of the EVR tied into the central return and the intake near the kitchen area. During the in between season, if the house were closed up and comfortable, I considered that I could flip the main switch off for the air handler and insert a "blank plate" into the filter slot so that the EVR could circulate using the duct system without running the AH or short circuiting the clean air. Is this likely to work?

    A. I'm not sure what you are aiming to accomplish. I advise anyone who wants to install an ERV to install dedicated ventilation ductwork rather than using HVAC ductwork to deliver ventilation air.

    Q. If there is long enough periods where the 470 CFM [whole-house exhaust] fan works to keep the house comfortable, would it be worth putting a screen on the sealed insulated crawl space door so that the air was being drawn through there before coming up to the living area?

    A. My guess is that any cooling effect due to drawing in crawl-space air would be short-lived. It wouldn't take long for the exterior air entering the crawl space to raise the temperature of the crawl space. Moreover, by introducing exterior air into the crawl space, you no longer have a sealed crawl space. Instead, you have a mold factory.

  45. Bob Ellenberg | | #45

    Combination system
    Martin, thanks for your prompt response.

    The idea of blocking the return filter in the HVAC system ductwork was so the ERV could utilitze the duct system during the time of year that the HVAC system was idle. I believe holding down cost should be a major goal of small energy efficient houses so more people will be able to afford them. My idea was to get good circulation without a dedicated duct system.

    Maybe I am simply ignorant, but I don't understand why the crawl space would have a greater tendency to have "mold". If it is a sealed insulated part of the buidling envelope that has air circulation normally. Bringing outside air through it as opposed to an open window above would seem to me to promote air circulation there during the season of the year when the central HVAC system is not being used.

    Again, thanks for your feedback and all the information you regularly share here.

  46. User avater GBA Editor
    Martin Holladay | | #46

    Condensation and mold in crawl spaces
    Bob,
    If you build a well-detailed closed crawl space with insulated walls and insulation over the dirt or gravel floor, and if you keep the house conditioned, and if the crawl space has a supply register connected to your forced air system, and if you heat your house in winter and cool your house with air conditioning in summer, then the surfaces in your crawl space will be at about the same temperature as the surfaces in your home upstairs.

    In that case, as you point out, there won't be much chance of condensation or mold in your crawl space.

    If I understand correctly, however, you are suggesting a crawl space without space conditioning (heat or cooling). It sounds like you want to flush the house with a whole-house fan at night. Let's say we're talking about the month of June. The only reason to use such a strategy is if the air conditioner is off. So now your crawl space isn't conditioned.

    Your crawl space will, indeed, be the coldest room of your house. Most crawls spaces lack insulation on their floors, so I don't know if you were planning to install insulation on the floor. If you introduce summer air into your crawl space, you'll get condensation on the cooler surfaces in your crawl space -- probably the poly covering the floor.

  47. Bob Ellenberg | | #47

    Condensation in conditioned crawl space
    Martin--sorry I was not clear.
    I am a builder/designer and the last personal home I built had the type of crawl space you describe except the floor under the membrane was not insulated--just the perimeter and it was in a dryer climate than where this house will be. My past experience with what I call the in between seasons was that I kept the house closed until late afternoon (possible only with excellent insulation and minimum glass exposure to the sun). As the walls and roof that had been heating all day in the sun began to heat the inside more, we slightly opened the windows and turned on the fan. There was usually a couple of hours where it was too warm (but not extreme) and then started to cool down. We would leave it that way until the next morning. The night air would have cooled it well by then and we shut the windows and fan off again. While living in that home I often thought that I should have installed an opening in the crawl space that would operate like opening a window since the crawl space was somewhat like being the lowest floor in the house and was always cooler.

    For this house I am planning exactly what you described, central HVAC, supply and return duct in the crawl space, etc. My question (and idea) about pulling air through the crawl space is a concept idea only for what I call the in between seasons where a tight well insulated house can be comfortable with neither heat or cooling operating. At such times I would sometimes ventilate using a small whole house fan as described above and sometimes have it closed up. My thoughts were that pulling air in through the crawl space would not only help the cooling factor but that it would actually help prevent any problems in the crawl space since there is no ventilation there at all when the central HVAC system doesn't run. My concerns revolve around the fact that this will be a hot humid climate. My personal experience has made me a strong proponent of a well executed conditoned crawl space. We will be using a stand alone dehumidifier in this house.
    Perhaps my basic question should have been are there potential problems with a conditioned crawl space in a more humid climate and?

  48. User avater GBA Editor
    Martin Holladay | | #48

    Response to Bob
    Q. Are there potential problems with a conditioned crawl space in a more humid climate?

    A. No, as long as you keep the crawl space sealed. Once you open a vent and let in the humid exterior air, however, all bets are off.

    "My thoughts were that pulling air in through the crawl space would ... actually help prevent any problems in the crawl space since there is no ventilation there at all when the central HVAC system doesn't run." But you're wrong. If you live in a hot humid climate, introducing exterior air into a crawl space causes the problems you are trying to avoid. That's why the crawl space needs to be sealed.

  49. Reuven | | #49

    HRV and ERV's
    What happens when you air seal an existing home to 0.2 ACH and then need to install mechanical ventilation? According to BPI, you must install mechanical ventilation to increase ACH to 0.35.

    Based on reduced ACH through air sealing the projected savings for the home is $265/yr. Does the installation of an HRV or ERV negate this savings benefit?

  50. User avater GBA Editor
    Martin Holladay | | #50

    Response to Reuven
    Reuven,
    You're making an apples-to-oranges comparison. Natural ACH is an estimate. Actual ACH due to air leakage through a leaky envelope varies widely. Leakage will be very high on windy days and on very cold days (due to the stack effect). On days with little wind or when temperatures are very mild, leakage will be much less, and the indoor air will get stuffy.

    If you install a mechanical ventilation system, your home will, for the first time, get a dependable source of fresh air, delivered every day of the year -- even when there is no wind or the stack effect is low.

    So let's say you decided to stick with natural air leakage. You end up with a house that often has poor indoor air quality, and on other days is over-ventilated. That's not what you want.

  51. Eric | | #51

    ERV vs Wave Home Solutions vs. Dehumidifier .
    My objectives are improved indoor air quality and lower summer humidity. I am in the Washington, DC area which is hot and humid in the summer and cold in the winter. I have a townhouse with storm windows and recently added energy efficient windows and doors. I have a heat pump heating and AC system and a heat pump hot water heater. I currently run a standalone small dehumidifier in the summer. If I'm away for 2 weeks in the summer with the AC and dehumidifier off, the basement will smell musty. Since I have asthma and allergies (especially dust mites), I'm concerned about indoor air quality. I also have standalone air purifiers on each floor. The basement is borderline on radon at 3-4.

    I'm finding it difficult to find objective information and am wondering whether it would be advisable to have an ERV, Wave Home Solutions (or competing product), and/or a whole house dehumidifier.

  52. User avater GBA Editor
    Martin Holladay | | #52

    Response to Eric
    Eric,
    Every tight home needs a mechanical ventilation system to bring fresh air into the house. It sounds like your house does not have one.

    If you read my article, you'll find that I survey several choices for providing fresh air to a home.

    Dehumidification is a separate issue from mechanical ventilation. If your air conditioner is incapable of controlling indoor humidity levels, then a stand-alone dehumidifier is a good choice.

    Radon control is also a separate issue. If you have radon concerns, you should contact a radon mitigation contractor.

    Finally, if you have special medical issues that made you nervous about your home's air quality after you return from a vacation, it may be worthwhile to operate your air conditioner and dehumidifier while you are gone. The price of two weeks' cooling may be worth it to avoid medical problems.

  53. Michael Isaacs | | #53

    Cheap Ventilation Solution...
    Hello,
    I've been trying to come up with a cost-effective and yet sensible ventilation solution as part of 1904 home remodel in Portland, OR. Been reading this and other stuff online and hope for some suggestions ... 2-levels, 1200sqft each. An unfinished basement(1200sqft) with a natural gas dryer venting out. Kitchen on main level includes a 700cfm hood. House came with 2 Rinnai NatGas Wall furnaces which i mean to keep. Top floor has a 48,000BTU output furnace which i will isolate to top floor vents, closing off 1st floor vents from this furnace.

    Main level is open with living, family, dining , kitchen space all in a continuum: one bedroom and attached bath are the closed off rooms with doors. Top floor has a large central space with doors leading to one bath and 3 bedrooms.

    My thoughts: one Panasonic ERV on each of 1st, 2nd stories in the open areas connected with a short duct run(<3ft) directly to outside air. Assuming the top floor erv counters the stacking effect. Add insulation to the attic. Panasonic Whispergreen 80cfm fan in 1st floor bathroom.

    Makeup air for kitchen hood: since the hood will be used intermittently (&very rarely at 700cfm) will a 18cfm passive inlet work? open a window? should i counter the basement dryer's effect to fight stacking? do i have to do something with the attic space... vague ideas of condensation in attic....
    thanks for any help
    Mike

  54. User avater GBA Editor
    Martin Holladay | | #54

    Response to Michael Isaacs
    Michael,
    1. The only way to reduce the stack effect is to seal the leaks in your home's thermal envelope. An ERV will not affect the stack effect, since it is a balanced ventilation system (air in = air out).

    2. Your home does not need 2 ERVs. One is plenty. Calculate your ventilation rate using the ASHRAE 62.2 formula.

    3. Yes, you can crack a window open to provide makeup air for your range hood, although a better solution is to replace your existing range hood with a much smaller model with a lower cfm rating. For more information on range hoods, see Makeup Air for Range Hoods.

  55. Jill Buffie | | #55

    Need reno advice please
    Wow, what a lot of great info! I am looking at my HVAC choices for our 1972 house in the BC interior. (mild winters, hot summers) It's 1300 sq ft, open concept and soon to be airtight with all new windows, doors and insulation. We are a small family of 3. My question is this: what are everyone's opinions about the HRV/ ERV debate where the heating/cooling is not supplied by furnace, but by an air to air heat pump exchanger? Can I go with the central fan integrated supply option?

  56. User avater GBA Editor
    Martin Holladay | | #56

    Response to Jill Buffie
    Jill,
    If you have an air-to-air heat exchanger, I assume that your space heat is delivered through ductwork. That means you can use a central-fan-integrated supply ventilation system if you want.

    If you prefer an HRV or an ERV, I recommend the installation of dedicated ventilation ductwork. If you are willing to compromise on energy efficiency, you can also distribute the ventilation air from an HRV or ERV through your space-heating ductwork, although such systems aren't as good.

  57. Jill Buffie | | #57

    HRV debate
    I will have ducts installed for the air exchanger. When deciding between the HRV or ERV, do things like floor plan (mine's open), family size (3) and sq ft-age (mine is 2700) make a difference?

  58. User avater GBA Editor
    Martin Holladay | | #58

    Response to Jill Buffie
    Jill,
    To find the answers to your questions, see HRV or ERV?

  59. David Butler | | #59

    Great article, but not so fast dis'ing central-fan-integrated...
    Late to the party, but someone just pointed me to this discussion. Really great article Martin. You covered a lot of bases that are not well understood by home performance practitioners.

    However, do do take exception to your characterization of the additional cost of ECM ($1000 to $1500). I'm surprised no HVAC dealers called you on that. My Lennox Elite ECM air handler costs about the same as the equivalent Elite model with PSC motor. In the case of a furnace, it's not the ECM motor that adds so much cost, it's that darned variable speed drive logic and multi-stage burner. Unfortunately, only a handful of manufacturers make single-stage ECM furnaces. Carrier's Boost 90 is one example. Write it down.

    OTOH, ECM air handlers are available from all the majors at very little upcharge (single speed, 5-tap X13 motor), and these models have identical efficiency as the equivalent variable speed models. As long as the duct system isn't restricted (e.g., blower operates in low-to-mid range of the cfm-watt curve), ECM blowers are far more efficient than their PSC counterparts. Unfortunately, too many contractors use these systems as a band-aide for undersized or otherwise restricted ducts, pushing the blower into the least efficient part of its performance curve.

    When operating and ECM blower for ventilation with a cycle controller, it should be set to operate on low. For example, my 3-ton AHU only consumes 105 watts on tap 1 (low).

    Bottom line, central-fan-integrated supply ventilation with cycle control is often a good choice. But as you point out, it must be properly designed and installed. But that's true for any ventilation system (don't get me started on poorly installed energy recovered ventilators!)

  60. User avater GBA Editor
    Martin Holladay | | #60

    Response to David Butler
    David,
    I wasn't disrespecting central-fan-integrated supply ventilation systems; I was merely alerting readers to the need to understand how they work and to be sure they are properly commissioned.

    I never wrote that an ECM blower costs $1,000 to $1,500 more than a PSC blower; rather, the problem is that furnace manufacturers (until recently, perhaps) have chosen to spec ECM blowers only on their high-end furnaces, not their entry-level models, so than anyone looking for a furnace with an ECM blower had to spend $1,000 to $1,500 more than they needed to. This was certainly the case when I wrote the article; if the situation has changed -- (and I do take note of your information on the Lennox Elite, although I'm not sure whether the Lennox Elite is affordable or pricey) -- then that's a good thing.

    Of course I agree with you that, as long as you keep your filters clean and design your ductwork properly, ECM blowers will save you energy and provide the best performance -- especially if you intend to use your furnace to distribute ventilation air during the swing seasons.

    Finally, you are 100% correct that any ventilation system -- including a system with an ERV or an HRV -- can be (and often is) poorly installed. (The worst systems I've seen are ERVs or HRVs hooked up to space-heating ductwork.) That's one reason I wrote the article -- to guide GBA readers and help them avoid problems.

  61. David Butler | | #61

    equipment models, price tiers
    Elite is Lennox's mid-tier product line (Merit is base, Signature is high). The model number for the ECM air handler is CBX27 (R410a). Dealer cost is about the same as the Elite CBX32 with PSC (CBX27 is actually slightly less for some sizes). Compared to the CBX26 in the Merit line, the CBX27 costs about $150 to $250 more, depending on size. I tend to specify mid-tier products in general.

    Carrier's Boost ECM furnaces are also mid-tier (Performance series) -- the Boost 90 58MEB (40k Btu/hr, single-stage) and Boost 90 58MEC (60k Btu and up, 2-stage). Carrier just introduced the Performance Series Boost 96 furnace -- the 59SP5 (40k Btu/hr and up, single-stage).

    I don't have ready access to Carrier prices, but I do know the Boost models cost substantially less than variable speed models, especially the 58MEB-040-12, which is large enough for most high performance homes. One thing I like about this model is it can handle up to 3 tons of cooling even though it's got the 40k burner. Getting enough blower capacity for a/c is often an issue with furnaces, often forcing an oversized model.

  62. Todd Oskin | | #62

    'Occupant-Sensing Ventilation' by conservation technology

    Any thoughts? Looks like the cost is getting up to be pretty close to a HRV/ERV...depending on the duct-work.

    Appears to be an exhaust only system, but with humidity and/or motion sensing monitors.. and with optional "air inlets" (see FRESH-AIR GRILLES) for super-tight buildings....

  63. User avater GBA Editor
    Martin Holladay | | #63

    Response to Todd Oskin
    Todd,
    The supplier is a reputable company, and I'm sure the system works well. However, I would withhold judgment on the energy-efficiency claim -- that this exhaust ventilation system operates at "the efficiency of central heat-recovery ventilation" -- until I read the same conclusion in a report from a third-party researcher. I don't know of any researcher who has performed such a study.

  64. L Buser | | #64

    House and crawl space ventilation in coastal marine climate
    Great forum. I enjoyed reading the article and letters. But I am left wondering about my situation.

    Here is where we are at right now:

    I have a 40-year-old split level house in the coastal marine climate of B.C., similar to Vancouver or Victoria. Winters rarely see snow but there is lots of rain and wind (up to 50 mph +), temps about 30-50 degrees F. Summer day times are about 70-85 degrees F and 60-70 percent humidity.

    The house is 1700 sq ft (excluding the crawl space) and about 17,000 cubic feet total, with the crawl space. There are two occupants. It is fairly well sealed.

    The foundation wall is concrete block about 30 inches high, with 24 inches below grade.

    The floor in the crawl space (front of house) and basement (rear) are at the same level and are concrete. Moisture isn't a problem. (As a precaution, I used a penetrating sealer on the crawl space floor and the basement floor varies from epoxy paint to ceramic tile to vinyl flooring).

    The basement wall was finished using standard framing, vapor barrier and fiberglass insulation (I may change it to xps and gyproc). The concrete block in the crawl space is exposed.

    What I want to achieve:

    My wife is sensitive to smells. The crawl space air gets drawn up into the house by the stack effect and the heating ducts (which aren't sealed) and I would like to eliminate that and add fresh air to the house with mechanical ventilation. At the moment we ventilate by opening windows. The high winter winds also add fresh air though air leakage.

    I have been considering a continuous-operation Panasonic fan, such as the variable speed 80 model that allows settings of 30-70 cfm. Would this be a good idea? I could also set it up with a programmable timer. It would draw air from the rest of the house to provide ventilation, remove the stale air from the crawl space and prevent the stale air from rising into the house.

    I was concerned that the fan could pull more vapor through the concrete block walls or through cracks in the 24 inch wood wall above the foundation, and if this could be a problem. I could cover the concrete block wall in the crawl space with eps (higher vapor transmission than xps).

    I considered adding a passive air intake in the crawl space to go along with the fan but this seems like it might not be a good idea from what I have read. Also, it would reduce the draw of air from the floors above.

    Other ventilation information:

    We have a new Lennox high efficiency furnace with ECM. I also had the Lennox Ventilation Control System installed (). It has a motorized damper and 6 inch diameter fresh air intake to the cold air return. It seemed like a good idea but I have been frustrated with it as the control is on the cold air return in the crawl space so it is not easy to change the settings. If you set it to run 15 minutes, it will run 15 minutes every hour, even if the furnace is not on. This results in cool air blowing through the floor ducts and causes a draft feeling of discomfort. As a result we don't use it at all.

    The heating ducts run through the crawl space and keep it at about 55 degrees F in the winter and 65-70% humidity. A hot air supply duct from the furnace could be added to raise that temperature but that would also increase the pressure in the crawl space so I don't know if that is the right thing to do.

    We have a new Panasonic 80 cfm in the upper floor bathroom and an older fan in the basement bathroom. There is a kitchen on the front (middle) floor and in the basement (but it is not used at this time). No stove fans (we don't fry food and open a window if necessary). No replacement air intake for the dryers (in the kitchen and basement kitchen).

    I hope I haven't overwhelmed you with details but I wanted you to have enough details to best assess the situation. Thanks.

  65. User avater GBA Editor
    Martin Holladay | | #65

    Response to L. Buser
    L. Buser,
    First of all, the best place to ask your question is on GBA’s Q&A page.

    If you post your question there, more people will see it, and you’ll be able to get answers from a wide number of people (including me).

    Briefly, however, here's what I suggest:

    1. The crawl space smell that bothers your wife is not a ventilation problem. It is a humidity problem. You need to address this problem in the crawl space. For more information, see: Building an Unvented Crawl Space.

    2. You need to seal air leaks between your crawl space and the conditioned space above.

    3. It is essential that you seal the seams of your duct system to make your duct system as airtight as possible. Here is more information: Sealing Ducts.

    4. You already have a ventilation system; however, it lacks an (also known as a FanCycler). Get one installed.

  66. Barbara A. Smith | | #66

    What about highly-filtered ventilation systems?
    Everybody here seems to be assuming that the outdoor air is not polluted with chemical sprays, wood smoke, exhaust fumes, or whatever. I knew a family that had to have a balanced ventilation system (no A/C necessary in the marine west-coast climate of the Bay Area) and the heating system was hydronically heated coils buried in the slab..

    What if a person is chemically sensitive and needs not only a HEPA filter but several hundred pounds of activated charcoal as well to deal with the aforementioned outdoor air pollutants? This family had a HUGE amount of ductwork for an approximately 3,000 square foot home and ran their ventilation-only system 12 hours a day. They had a pre-filter, 1,000 pounds of activated charcoal, and a HEPA filter. They did not (in my mind) live in a particularly polluted area, but they could not predict when their neighbors might decide to use their fireplaces.

    John Bower, who wrote the book, Healthy House Building for the New Millineum, was able to get away with a only a HEPA filter a a few pounds of activated charcoal. They lived in bedroom community where their neighbors were gone during the day and thus not pollute the outdoor air with wood smoke. But if you can't predict your neighbors' pollution, or if you live in a polluted almost-all-the-time area as I do, and are chemically sensitive to boot, what's the most energy efficient way to ventilate? Obviously, a dedicated ventilation system is the only way to go, and unfortunately, most builders are not familiar with them.

  67. User avater GBA Editor
    Martin Holladay | | #67

    Response to Barbara A. Smith
    Barbara,
    Thanks for your comments. Here's my reaction:

    1. Many studies have confirmed that, on average, the indoor air in U.S. homes is much more polluted than outdoor air. This is true even in urban areas that suffer from air pollution. There are a huge number of possible pollutants in most U.S. homes; these include humans (who have viruses, who give off odors, and who produce water vapor), dogs, cleaning products, the byproducts of cooking, tobacco, flame retardants from sofas and armchairs, and chemicals in carpeting. I don't doubt that you have attempted to minimize these emitters, but nevertheless, indoor air is usually more polluted than outdoor air.

    2. If HEPA filters and charcoal filters help your symptoms, then you should continue to use them, of course. However, no one should install these devices on their ventilation systems unless someone in their home has medical issues, because HEPA filters and charcoal filters introduce static pressure in the ventilation system, and require the use of much more powerful fans to push air through the thick filters. As a result, these filters incur a major energy penalty, and your electricity bill is higher than it would be if you didn't have these medical symptoms.

  68. Phil Lawson | | #68

    Ventilation
    In a really tight house (< 1 ACH @ 50), or a passive house, you can reduce your heating load to next to nothing with a high efficiency HRV/ ERV (ie Zehnder or Ultimate Air) but at higher cost (1500-2500 for base unit). A new system coming out (CERV from ) solves a number of problems with one unit that includes:
    - the heat (no more minisplit in your wall) as it integrates a heat pump similar to a minisplit and has a back up electric coil heat for very cold periods
    - the HRV component
    and a few features that outperform efficiency wise: demand ventilation by remote switches; recirculation of air or bringing in fresh outside air with high efficiency depending on VOC or CO2 levels; and automatic on and off depending on interior CO2 and VOC levels. Instead of constant operation (like most high efficiency HRV's) the unit runs when needed and balances air to preset (via a central control unit) levels that can be adjusted (ie VOC or CO2 levels in the range of 800-1000 ppm).
    The outcome is HRV, central heat and central AC in one ducting system with no real need for supplemental heat in a really tight well designed passive or other high performance house. The cost is about the same as a minisplit and high end HRV/ERV..... and this is the first US made available high end system of this sort at a reasonable cost that I am aware of.....

  69. User avater GBA Editor
    Martin Holladay | | #69

    Response to Phil Lawson
    Phil,
    You wrote, "In a really tight house or a passive house, you can reduce your heating load to next to nothing with a high efficiency HRV/ ERV (ie Zehnder or Ultimate Air)."

    I'm sorry, but you are wrong. An HRV or ERV will not reduce your heating load to next to nothing. Running an HRV or ERV always increases your heating load -- it never reduces your heating load. The more you ventilate, the higher your heating bill.

    What reduces your heating bill is the thick insulation and the air-tightening measures. Ventilation always increases your energy cost.

  70. Rick Martelli | | #70

    exhaust only ventilation
    Hi Martin and Everyone,

    I know you may have stated that exhaust only ventilation may be a decent option for smaller homes with open floor plans. I have a single level home with a walk out basement. Each floor has roughly 800 to 900 sqft. Each level is very open with the exception of the bedrooms obviously. I was planning on running 2 Panasonic fans in the bathrooms in the lower level and a Panasonic fan in the bathroom in the main level. You had stated that it may not be necessary for the installation of passive air which would be great. However, the home is getting spray foamed on all perimeter walls and up to the peak of the roof due to the cathedral ceiling. There will be no ridge or soffit venting. Given the complete sealing of spray foam, do you still feel it would be unnecessary for me to install passive venting? I was considering those Lunos fans but they are quite pricey

  71. User avater GBA Editor
    Martin Holladay | | #71

    Response to Rick Martelli
    Rick,
    First of all, you don't want to run three Panasonic fans continuously, because they will probably overventilate your house. But you probably know that.

    As long as your fans are controlled by timers to prevent overventilation, I think you'll be fine.

    There's a simple way to determine whether your house has enough air leakage for an exhaust-only ventilation system: measure the exhaust fan flow when the fans are installed. This process, called "commissioning," should be a routine part of any fan installation, as I explained in the last section of my article.

    If your fan is rated at 60 cfm, and you can measure that it is exhausting air at that rate (or near enough to that rate to meet your expectations), then clearly there is enough makeup air. Air in = air out.

  72. John Bailey | | #72

    venting into the duct system
    Hi folks, having read the discussion I am left thinking about providing outside air to the duct system. This could be for example a 100cfm fan supplying the ducts through a backflow damper. I realize that the flow through the ducts would be very slow, but the volume would have to travel through somewhere. So I wonder why this would not solve the problem of the 800 Watt motor and provide pretty good distribution.

  73. User avater GBA Editor
    Martin Holladay | | #73

    Response to John Bailey
    John,
    The usual method for the type of ventilation system you describe is explained in the article. This type of ventilation system is called a "central-fan-integrated supply ventilation system." It requires a motorized damper, a FanCycler control, and a furnace or air handler with an ECM blower.

    Adding another fan (as you propose) doesn't simplify things; it complicates things. You need to prevent the air handler fan from pulling outdoor air through your proposed outdoor air duct -- how will you do that? You need a control that manages the operation of the ventilation fan so that it doesn't run at the same time as the furnace fan -- how will you do that? You need to come up with a way to ventilate the house when the furnace fan is running for 12 hours straight -- how will you do that?

  74. John Bailey | | #74

    more venting into the duct system
    Oh, I see there are issues I hadn't thought of.

    Maybe my own situation is peculiar. I have a FHA system I do not use because I have a lovely radiant floor for winter. So my ducts are idle. It seemed to me venting into those for the eleven months of the year we do not use AC could be a decent venting solution.

    But it does seem that anyone should be able to vent to the high pressure side of the ducting as long as they have a damper.

  75. User avater GBA Editor
    Martin Holladay | | #75

    Response to John Bailey
    John,
    You wrote, "It does seem that anyone should be able to vent to the high pressure side of the ducting as long as they have a damper."

    Reread the article. For this system to work, you need more than a duct from the exterior and a (motorized) damper.

    The FanCycler control (also called an AirCycler control) is essential; it shuts the motorized damper when necessary to prevent overventilation, and energizes the furnace fan when necessary to prevent underventilation.

    The outside air duct should be connected to the return air plenum, not the supply air plenum.

    The above comments are in response to your statement that "anyone should be able to vent to the high pressure side of the ducting as long as they have a damper."

    If we are talking about your specific case -- rather than "anyone" -- then you can rig up any kind of ventilation system that works for you. If you want continuous ventilation at a steady rate, you probably don't need any control more complicated than a toggle switch, as long as (a) you know how to measure airflow so that you can verify that your fan isn't overventilating, and (b) you have a good way of shutting the outdoor air duct tightly during the air conditioning season, and (c) you have another strategy for ventilation when you are operating your air conditioning system.

  76. Venkat Y | | #76

    Expelling of indoor contaminant with supply ventilation
    I have condensation on windows in my ICF home in the winter, which I guess means I have high humidity. If I employ supply ventilation using an AirCycler to bring fresh dry air from the outside, I am wondering where will the moisture-laden air already in the home go? TIA.

  77. User avater GBA Editor
    Martin Holladay | | #77

    Response to Venkat Y
    Venkat,
    All homes have leaks in their thermal envelopes. Air is always entering your home through cracks (this is infiltration) and also leaving your home through cracks (this is exfiltration).

    If you install a supply-only ventilation system, fresh air is delivered through your HVAC system, slightly pressurizing your house. Air leaves your house the same way it always does -- through cracks and holes in the thermal envelope.

  78. Venkat Y | | #78

    Martin,
    Thanks for the

    Martin,

    Thanks for the response. When you say all homes have leaks in their thermal envelopes, do those include tight ICF homes like mine that normally don't allow moist air to escape? Even then I take it the moist air in my home quite won't exit in amounts anywhere comparable to that taken out by a balanced ventilator?

    Thanks again.

  79. User avater GBA Editor
    Martin Holladay | | #79

    Response to Venkat Y
    Venkat,
    Q. "When you say all homes have leaks in their thermal envelopes, do those include tight ICF homes like mine?"

    A. Yes. All homes have measurable rates of air leakage. That's why we test homes with a blower door -- so that we can measure the rate of air leakage.

    Q. "I take it the moist air in my home quite won't exit in amounts anywhere comparable to that taken out by a balanced ventilator?"

    A. The volume of air exiting your home is always exactly the same as the volume of air entering your home. (Otherwise, your house would blow up like a balloon and explode.) If your supply ventilation system is introducing 100 cfm into your home, then 100 cfm is escaping through envelope cracks.

  80. Timothy Godshall | | #80

    random cracks vs. passive inlets & summertime humidity
    I am designing a house with an exhaust-only ventilation system (2 Panasonic bath fans on timers) and have read the debate between relying on random cracks and passive inlets to supply make-up air. I like the simplicity of not having to install passive inlets, but I wonder about the effect of drawing humid summer air through a wall into a cool house. Isn't that a recipe for condensation on the cool, inside surface of the walls? Even if you install passive inlets, wouldn't moisture condense in them, too?

  81. User avater GBA Editor
    Martin Holladay | | #81

    Response to Timothy Godshall
    Timothy,
    The short answer to your question is that you shouldn't worry.

    Good building envelope design goes a long way to avoiding problems.

    The most likely surfaces to experience summertime condensation are metal air-conditioning ducts and metal ceiling registers or duct boots. If you can keep outdoor air away from these surfaces, you should be OK.

    The fact is that your walls experience infiltration of exterior air whether you want them to or not. The main driving force for this infiltration is wind. Wind finds your wall's cracks, and wind pressures greatly exceed the pressures exerted on a wall by a ventilation system. Almost all of the humidity introduced by these wind pressures is harmlessly absorbed and harmlessly evaporates.

    If exterior air enters passive air inlets during the summer, the temperature of the incoming air raises the temperature of the metal or plastic grille, so condensation is unlikely.

  82. Timothy Godshall | | #82

    response to Martin
    Thanks, Martin. I'm glad to check that off my list of worries!

    I am curious to know, though, why the infiltration of warm humid air in the summer is of little concern but the exfiltration of warm humid air in the winter is. (You probably recognize my name as I have been posing questions about that problem on another discussion thread.) The only thing I can think of is that the indoor vs. outdoor temperature differential in climate zone 4 is not typically as great in the summer as it is in the winter, so there's not as much likelihood of condensation. Is that it, or are there other reasons?

  83. User avater GBA Editor
    Martin Holladay | | #83

    Response to Timothy Godshall
    Timothy,
    Q. "Why is the infiltration of warm humid air in the summer of little concern, but the exfiltration of warm humid air in the winter is?"

    A. First of all, in winter the temperature of cold surfaces can be much lower than the temperature of surfaces in summer. In the area of Vermont where I live, it's possible for wall sheathing or roof sheathing to be at 0°F or even -20°F. Air conditioning ducts never get that cold. And gypsum wallboard never gets much colder than 70°F in the summer.

    Second, the stack effect is a very powerful driver, and the stack effect depends on delta-T. Wintertime delta-Ts are much higher than summertime delta-Ts.

    It's possible to get summertime condensation problems (for example, when inward solar vapor drive is the driving force, and a builder chose to install a reservoir cladding, a vapor-permeable wall sheathing, and interior poly or vinyl wallpaper) -- but you have to make more mistakes for these things to happen than you do to get wintertime condensation problems.

  84. Ryan Hagerty | | #84

    Plusaire product question
    Martin,I have found a Canadian ventilation product called Plusaire. It looks like a passive ventilator as well as being attached to the powered furnace. Is this a viable option in the field of ventilation? I am considering ventilation for my 1970s era house. Thanks

  85. User avater GBA Editor
    Martin Holladay | | #85

    Response to Ryan Hagerty
    Ryan,
    This ventilation box seems to be designed to serve a central-fan-integrated supply ventilation system. However, it seems to lack a motorized damper. Without a motorized damper, it appears that the Plusaire box can allow a lot of exterior air to enter your home -- in other words, it can over-ventilate.

    If you want a central-fan-integrated supply ventilation system, I suggest that you use the components mentioned in my article (a motorized damper and an AirCycler control), not a Plusaire appliance.

    .

  86. Ryan Hagerty | | #86

    Response to Martin
    Thanks for the comments Martin
    Can a damper and AirCycler be used stand alone with an existing central air system? The videos I have seen seem to have dedicated fresh air input ducts. Would a system such as this solve back drafting problems in an oil furnace? I had a blower door test yesterday and some low levels of CO was detected.

  87. User avater GBA Editor
    Martin Holladay | | #87

    Response to Ryan Hagerty
    Ryan,
    Q. "Can a damper and AirCycler be used stand alone with an existing central air system?"

    A. I'm not sure what you mean. All central-fan-integrated supply ventilation systems are installed in homes with a forced-air HVAC system (ductwork connected to a furnace or an air conditioner). If you have forced air ductwork, then your house is a candidate for a central-fan-integrated supply ventilation system.

    Q. "The videos I have seen seem to have dedicated fresh air input ducts."

    A. I don't know what video you are talking about. But it is certainly true that a central-fan-integrated supply ventilation system requires an outdoor air duct. The duct conveys outdoor air to the return plenum of the furnace or air handler. Somewhere in that duct run, you need to install a motorized damper controlled by an AirCycler.

    Q. "Would a system such as this solve back drafting problems in an oil furnace?"

    A. No. Backdrafting is usually caused by powerful exhaust appliances (for example, range hood fans). For more information on backdrafting, see Makeup Air for Range Hoods.

    The two most common solutions to backdrafting problems are: (a) replace the backdrafting appliance with a sealed-combustion appliance, or (b) introduce outdoor makeup air to a location near the backdrafting appliance, via a passive outdoor air duct or a makeup air appliance.

    A ventilation system is different from a makeup air system. So far, we've been talking about ventilation systems, not makeup air systems.

  88. Ryan Hagerty | | #88

    Response to Martin
    Thanks for your patience Martin,

    The video I looked at was from the AirCycler webpage. Now that I look back, it was for a "G2 whole house ventilation system" not just the AirCycler. I am looking for something easily retrofitted into an existing forced air HVAC system.

    As to my oil furnace backdrafting comment, I was confused as to the difference between make up air and ventilation. Since both ventilation and make up air are introducing fresh air, can one solve the other at times?

    I had an energy audit recently and the auditor found low CO levels near the oil furnace. I didnt have any fans running, or any powered air usage going on at the time of the readings. The auditor found a substantial air leak behind the chimney which I have filled. My other suspicion is the chimney itself being very cold and larger than the pipe going into it. So whether it be make up air, ventilation or a chimney liner, I am hoping to solve the CO problem.

    Thanks for your help!

  89. User avater GBA Editor
    Martin Holladay | | #89

    Response to Ryan Hagerty
    Ryan,
    Q. "I was confused as to the difference between make up air and ventilation. Since both ventilation and make up air are introducing fresh air, can one solve the other at times?"

    A. No. A ventilation system is designed to introduce fresh outdoor air for occupant health (and sometimes, to lower indoor humidity levels). In most cases, ventilation systems exhaust stale air while introducing fresh air.

    If you have a backdrafting problem that requires the installation of a makeup air system, that's what you should install. A makeup air system has different design requirements and details from a ventilation system. Moreover, almost all ventilation equipment manufacturers note that their equipment is not intended to provide makeup air.

    Q. "I had an energy audit recently and the auditor found low CO levels near the oil furnace. ... My other suspicion is the chimney itself being very cold and larger than the pipe going into it."

    A. This venting problem is a potential safety hazard. Since your energy auditor evidently failed to offer you useful advice on how to solve the problem, you need to hire a combustion expert to inspect your venting arrangement and recommend a solution. A gas company or a contractor who specializes in oil furnaces or oil boilers should have enough expertise in venting issues to provide a solution. Don't delay -- fix this promptly.

  90. Marion Solen | | #90

    inadequate fresh air ventilation for power vented gas HW heater
    We recently purchased a property that is just under two years old - it is located on 3rd and 4th floor of a structure that was built from scratch and has power vented hot water heater on the 4th floor supplying our property. I noticed that it seemed to be running constantly and shutting on and off and that it took a long time for hot water to regenerate. I am a Realtor and have a small amount of experience and I was concerned that the ventilation was inadequate. My plumber came and followed the Bradford White flow chart and confirmed that my suspicions were correct. In fact, when we disconnected it from the fresh air intake and just used room air - the hw heater operates just fine.
    Problemis what to do now? We think the width of the pipe is too narrow - it goes a fairly long distance and has a couple of tight turns as well. Also, it is in a finished ceiling in a bedroom before it goes outside (we also see stains on the ceiling there that I thought was a roof leak -however, ever since disconnecting this pipe - the ceiling stains have not worsened despite heavy rains - we think there may have been actual leaks and/or condensation dripping into the ceiling - no way to know. The builder (is also a neighbor) says there was a one year warranty which is over (however- do not think this should apply to improper construction). At any rate, is there a way to increase ventilation or completely avoid using that pipe. We have a very open floor plan although that hot water heater is in a closet - we can get additional air in there by opening a hole where there is already a grill for fresh air return into the HVAC which is installed in the same closet.
    Bottom line - is it OK to use this hot water heater without connecting to the fresh air intake?

  91. User avater GBA Editor
    Martin Holladay | | #91

    Response to Marion Solen
    Marion,
    This question has nothing to do with mechanical ventilation systems -- the subject of this article. Rather, your question has to do with providing combustion air for your water heater.

    I don't know what type of water heater you have. It may be a sealed-combustion water heater (one that requires ducted outdoor air) or it may be a power-vented water heater (one that can be supplied by unducted room air).

    In any case, it seems clear that the outdoor air duct is defective. Either the diameter is too small; or there are too many elbows; or the seams are leaky; or all of the above.

    If diagnosing and fixing this problem is beyond the capabilities of your plumber, you should contact a technical representative from Bradford White.

  92. Adam W | | #92

    Dedicated HRV design question
    New subscriber here - is there a GBA article on how to layout the supply and returns of an HRV system? As in - bedrooms all get supply vents, laundry and kitchen get returns, etc...

  93. User avater GBA Editor
    Martin Holladay | | #93

    Response to Adam Wride
    Adam,
    Briefly, the living room and bedrooms get the supply registers, and the bathrooms, laundry, and kitchen get exhaust grilles. (Note that the ceiling-mounted kitchen grille should be located far from the range.)

    Your should follow the instructions of the HRV manufacturer, of course. Note that installation methods that share ducting with a forced-air heating system are inferior to installation methods using dedicated ventilation ductwork.

    For more information, see .

    I also recommend these two articles:

    Does a Home with an HRV Also Need Bath Fans?

  94. Timothy Tucker | | #94

    Exhaust only with 2 bathrooms
    Considering going with an exhaust only approach on the mid-90's home that we just purchased. It's a long way to go to get to the point where we likely need to be too worried about exhaust (lots of sub-par sealing and insulating, including a gaping hole for the whole house fan), but any feedback to help with planning would be appreciated.

    There's a larger first floor bathroom (bath 1), a 1/2 bath on the opposite side of the house (bath 2), and an upstairs full bath (bath 3). (Brief view of the floor plan attached).

    The fan for bath 2 has flex duct running from the fan into the attic area above -- unsure how difficult the run between the floors would be to replace, or if it'll even be possible to replace.

    Bath 1 would have its own fan on a switch / timer.

    My current thought is to use a Panasonic inline fan to serve baths 2 & 3, connected to a switch / timer in bath 3, with no switch in bath 2. (So the exhaust would pull from both rooms whenever the upstairs bath was in use or when the timer indicates recirculation is needed).

    Note that bath 3 also has a laundry chute connecting to the laundry room below, so option 2 to exhaust stale air from the downstairs would be to change to a louvered door for the chute and possibly add a jump duct between the bathroom and laundry room. At which point, I might just leave the existing cheap exhaust fan in bath 2 alone.

  95. User avater GBA Editor
    Martin Holladay | | #95

    Response to Timothy Tucker
    Timothy,
    If you are going to be installing bathroom exhaust fans in three bathrooms, I think that each bathroom should have a separate fan, and each fan should be controlled by a switch located in the bathroom it serves. Of course, some of these fans can also be on timers; a typical approach is to have continuous operation at a low fan speed, with the option of a higher fan speed when desired (often by flipping a toggle switch). The low-speed operation can either be continuous or timed (20 minutes on and 40 minutes off, for instance).

    If you haven't read them yet, I urge you to read these two articles:

    Bathroom Exhaust Fans

    Is Your Ventilation System Working?

  96. Timothy Tucker | | #96

    At this point, there are fans
    At this point, there are fans already present in each bathroom, the debate is what to change.

    The consensus that I saw from the Bathroom Exhaust Fans article seemed to be that there wasn't a need for an exhaust fan in a small bathroom with no shower (which is the case for bath 2).

    Here's a clearer idea of the options that I'm looking at:
    1. Remove the fan from bath 2 to eliminate an extra hole in the ceiling that isn't really needed. Bath 3 gets an upgraded to an quieter inline fan connected to a timer for exhaust. (Given that the ceiling is textured and would be difficult to patch without looking odd, this option is probably a non-starter).

    2. Leave the setup for bath 2 as-is, with a cheap but noisy fan that isn't likely to see much use. Bath 3 gets an upgraded to an quieter inline fan connected to a timer for exhaust. Door to the laundry chute gets replaced with something that will allow some air from the laundry room below to be exhausted when venting.

    3. Bath 3 gets upgraded to an quieter inline fan connected to a timer for exhaust. Bath 2 is also connected to the same inline fan for exhaust. Likely requires a higher CFM fan than option 2, since more duct work would be connected and increase static pressure, but would possibly give better ventilation for the first floor areas near the bathroom.

    Would you agree that option 2 sounds like the best bet?

  97. User avater GBA Editor
    Martin Holladay | | #97

    Response to Timothy Tucker
    Timothy,
    Half baths (bathrooms without a shower or tub) need an exhaust fan for odor control. So whatever you do, don't leave that room without a fan.

    Inline fans are fine, but you don't need to switch to an inline fan to get a quiet fan. Panasonic makes lots of ceiling-mounted fans that are extremely quiet -- so quiet that many people don't even realize they are on.

  98. Danny O | | #98

    Our ventilation system
    Our ventilation system suffers from most of the issues you mentioned, and I'm trying to figure out how to improve the situation. We have two ERVs (basement and unconditioned attic) running at ~ 120 cfm each (big house that requires about 240 cfm according to ASHRAE calculations). They are mounted to our home return ducts. Because the ERVs are small, we run them at their maximum flow rate 24 hours a day in winter and summer, which entails a large energy penalty because the two furnace fans are also running constantly to propel the ventilation air. The efficiency of these ERVs is not great (heat and moisture transfer), and in the winter, the air coming out of the registers is pretty cool during the part of the cycle when the furnace isn't heating the air. When we were designing the system with the HVAC contractor, we were cautioned to avoid adding a dedicated duct system for the basement ERV because it would require an expensive retrofit. We could have more easily put a dedicated ERV duct system in the attic, but because the attic is unconditioned, we were told that this would be problematic. So we stuck with using the existing furnace ductwork. I'm wondering if we could at least partially address the comfort and energy efficiency issues we're experiencing by 1) stopping use of the attic ERV altogether and 2) replacing the basement ERV with a commercial ERV running at around 600 cfm mounted to the return plenum that would only run when the furnace heat was also running (so we'd still be getting close to 240 cfm of ventilation, on average, but not continuously)? I don't know if the combined flow of the furnace fan (~1000 cfm) and such an ERV (600 cfm) would cause problems. The lower duct system (which provides air to the basement and main level of the house, but not the upstairs) is pretty leaky (but only leaks inside the house, I'm told), so I don't know if air resistance would be an issue. I also don't know if that large volume of ventilation air would be heated sufficiently by the furnace. I'd be interested in your thoughts on this idea and any other suggestions you might have. Thanks.

  99. User avater GBA Editor
    Martin Holladay | | #99

    Response to Danny O
    Danny,
    I don't recommend the use of an ERV if the ERV uses forced-air ductwork connected to a furnace with an energy-hog fan. This is a bad system.

    Before making any decisions, the best step would be to hire a home-performance contractor or energy rater capable of measuring ventilation system air flow. (This article explains various methods of measuring air flow: Is Your Ventilation System Working?)

    When the furnace fan is running, the furnace fan may be helping pull extra air through your ERV. As a result, you may be ventilating at a much higher rate than you think.

    240 cfm is a lot of ventilation (and of course your current setup may be delivering much more). Even though that's what ASHRAE 62.2 recommends, I wonder whether it's necessary. How many people live in the house?

  100. Danny O | | #100

    Our Ventilation System
    Martin, thanks for your response and recommending the article, which I found interesting.

    When we had the ERVs installed, we used a flow hood outdoors to measure the rate of fresh air being sucked through each intake mounted on the outside of the house. The flow into each intake was about 120 cfm. Is it correct to assume that this is the amount of fresh air entering the house? The contractor also tested the flow for one system by inserting a small probe in the fresh air duct just before it entered the return plenum—this value was close to the flow rate measured using the flow hoods. Are these the correct ways to measure the ventilation system airflows?

    Four people live in the house. The ASHRAE calculation is based on a home energy auditor’s testing (including blower door test and home volume calculations). The house was pretty tight to begin with, and the auditor sealed the attic floor (among other things) to make the house even tighter.

    Your recommendation to avoid using forced-air ductwork makes sense to me. So, if we had the opportunity to start over, would you recommend adding a dedicated ERV duct system from the basement, even though the retrofit would be expensive? Would you put a dedicated ERV duct system in the unconditioned attic (or first seal the attic walls/ceiling to make it a conditioned attic)? Alternatively, we could have installed a motorized damper controlled by an AirCycler, but wouldn’t that also require use of the forced-air ductwork and have an even higher energy penalty than our current system? Plus, the AirCycler (or our existing ERVs) wouldn’t be able to get close to 240 (or 120) cfm if running discontinuously and limited to 7% of total furnace air flow. I’d appreciate any advice you have. Thanks!

  101. User avater GBA Editor
    Martin Holladay | | #101

    Response to Danny O
    Danny,
    Air flow through your ERV probably varies depending on whether the furnace blower is on or off. That raises two questions:

    1. Did the HVAC contractor wire the ERV controls so that the furnace fan comes on whenever there is a call for ventilation?

    2. When the ventilation flow rate was measured, was the furnace fan operating, or just the ERV fans?

    For more information on ducting ERVs, see .

    It is not true that "The ASHRAE calculation is based on a home energy auditor’s testing (including blower door test and home volume calculations)." As stated in this article ("Designing a Good Ventilation System"), the ASHRAE formula is based on the number of square feet of occupiable space in the house and the number of occupants.

  102. Danny O | | #102

    Our Ventilation System
    I wondered about that too after your last post, but I don't think we have higher actual ventilation rates than those that were measured. The HVAC contractor wired the ERVs so that they turn on whenever the furnace blower fan turns on. So, if the fan setting on the thermostat is set to "Auto", the ERVs turn on only when there is a call for heat or AC. If the thermostat is set to "Fan On", the ERVs will run continuously. Since the ERVs are only on when the furnace fan is on, the ventilation flow rate had to have been measured with the furnace fan and ERVs operating together. In fact, the flow rate was measured repeatedly over several days and only when the fan setting on the thermostat was set to "Fan On". Any thoughts about the other questions from my last post?

  103. User avater GBA Editor
    Martin Holladay | | #103

    Response to Danny O
    Danny,
    Q. "Your recommendation to avoid using forced-air ductwork makes sense to me. So, if we had the opportunity to start over, would you recommend adding a dedicated ERV duct system from the basement, even though the retrofit would be expensive?"

    A. Yes.

    Q. "Would you put a dedicated ERV duct system in the unconditioned attic (or first seal the attic walls/ceiling to make it a conditioned attic)?"

    A. It's always a bad idea to install an ERV in an unconditioned attic. Have you contacted the ERV manufacturer to see if this type of installation follows the manufacturer's installation instructions? In some climates, an attic can get quite cold during the winter -- cold enough, I would imagine, to add to icing problems. So, to answer your question: An ERV belongs inside the conditioned space of the building.

    Q. "We could have installed a motorized damper controlled by an AirCycler, but wouldn’t that also require use of the forced-air ductwork and have an even higher energy penalty than our current system?"

    A. The type of ventilation you describe as "a motorized damper controlled by an Air Cycler" -- usually called a central-fan-integrated supply ventilation system -- usually has higher operating costs than a well-installed ERV system. But you don't have a well-installed ERV system. If you decide to install a central-fan-integrated supply ventilation system, you need to make sure that your furnace has an ECM blower -- or else you'll run into the same problem you have now (high energy costs due to an inefficient furnace blower).

    If there are four people living in your house, then the main driver for your very high ASHRAE 62.2 calculation (240 cfm) must be the fact that your house is very large. One approach is to design an 80-cfm ventilation system that serves the rooms you actually use (perhaps your living room and bedrooms), and then to install a rarely used ventilation system (equipped with an on-off switch) serving the rooms in your house that are usually empty.

  104. Danny O | | #104

    Our Ventilation System
    For the 80-cfm ventilation system you mentioned, would it be reasonable to install something inexpensive like this Panasonic FV-04VE1 WhisperComfort™ Spot ERV in each upstairs bedroom (running the pair of ducts through either the attic or an exterior wall to get to the outside)? Would this approach be similar to the Lunos ERVs (which are significantly more expensive)? It seems like this would be a relatively cheap way to take advantage of a dedicated ERV duct system without an expensive retrofit or having to condition the attic.

  105. User avater GBA Editor
    Martin Holladay | | #105

    Response to Danny O
    Danny,
    Where do you live? The Panasonic ERV can't be used during the winter in cold climates.

  106. Danny O | | #106

    Hi Martin, I live in
    Hi Martin, I live in Washington, D.C. Is this ERV OK, or is there a different unit that would work better? Thanks.

  107. User avater GBA Editor
    Martin Holladay | | #107

    Response to Danny O
    Danny,
    According to Panasonic, in Washington, DC, the ERV that you are thinking of using will only work from March through November. It cannot be operated in December, January, or February. For me, that would be a deal-killer. You have to decide whether that level of performance makes sense for you and your family.

    For more information, see the installation manual. Here is the link:

  108. Danny O | | #108

    Yes, that's a deal-killer for
    Yes, that's a deal-killer for this unit. Based on Panasonic's operation season map, we'd either have to move south or west to use this unit (or wait for global warming to bring Washington, D.C. into the geographic zone where this unit can be used year-round!).

    Thanks very much for your help, Martin. I'll keep an eye out for similar types of units that might come on the market that can be used year-round where we live.

  109. Danny O | | #109

    Air Quality Monitoring
    Hi Martin, what do you think of the various residential units for monitoring air quality (e.g., VOCs, CO2, particulates, mold), like the ones in this link?

    Do you recommend any of these or others (either residential or commercial units)? Is it better to hire a professional? Does is make sense to compare indoor to outdoor air quality?

  110. user-7075735 | | #110

    Hi Martin,

    I've read many of your articles and posts regarding a central-fan-integrated supply ventilation system, but I don't recall seeing an explanation to one question. In this type of system how is air leaving the house to allow the central-fan to bring in air from a fresh air intake duct connected to the return plenum?

    Does this system rely Solely on air being positively pressured out bathroom and kitchen exhaust fans and miniature cracks in the building envelope?

    If so can you roughly estimate the amount of air being positively pressured out of the house to determine how much fresh air you're bringing in?

    Thanks,
    Arden

  111. User avater GBA Editor
    Martin Holladay | | #111

    Arden,
    A central-fan-integrated supply ventilation system depends on random envelope leaks for the exhaust path -- just as a supply-only ventilation system depends on random envelope leaks for the fresh air path.

    To determine the rate of air flow provided by a central-fan-integrated supply ventilation system, the installer needs to measure the airflow through the supply duct. Air flow rate out equals air flow rate in -- that's an inviolable rule.

    For information on ways to measure air flow through a duct, see "Is Your Ventilation System Working?"

    1. user-7075735 | | #112

      Martin,
      I have a cabin in Saskatchewan, Canada where the extreme winter temperatures can get pretty cold i.e. -40 degrees, similar to Northern US states. I like the premise of the central-fan-integrated supply ventilation system. I want to filter the fresh air and distribute it effectively. However, I'm a little concerned about exhausting air into envelope leaks. Are you not worried about this warm, humid air causing condensation issues behind unknown places in walls during cold winter periods?

      You've referenced the AirCycler control in your articles and I was wondering if you would recommended using the FanConnect option which syncs a bathroom exhaust fan with the motorized damper and the central fan call. This is supposed to provide a direct exit for exhaust air rather than pressuring it into envelope leaks. Does this cause an unwanted issues besides extra electricity for the bathroom fan when it's engaged by the FanConnect control?

      Also, when connecting an insulated fresh air intake duct to the cold air return plenum is there any special installation methods to reduce the chance of condensation either in the fresh air intake duct or where the cold, fresh air meets the cold air return plenum metal. Most days I can't see this being an issue in my climate, but on stretches of really cold temperatures I'm concerned.

      Lastly, with a central-fan-integrated supply ventilation system how do you bring in enough make-up air in the event multiple exhaust fans were running ex. bathroom exhaust fan, dryer, and kitchen exhaust fan. Say this added up to 400 cfm. In one of your articles you gave an example of measuring the incoming airflow in order to ensure it was ~10% of the total return air so you could ensure the return air temperature remained above a certain threshold before passing by the heat exchanger in the furnace. I believe in your example the incoming air flow was 100 cfm. Is there a way to deal with specific situations where the incoming air flow needs to be 4 times greater while still being mindful of duct condensation and heat exchanger air temperature issues?

      Thank-you,
      Arden

  112. User avater GBA Editor
    Martin Holladay | | #113

    Response to Comment #112:
    Arden,
    Q. "I'm a little concerned about exhausting air into envelope leaks. Are you not worried about this warm, humid air causing condensation issues behind unknown places in walls during cold winter periods?"

    A. The answer to your question can be found in my article, where I wrote, "Some builders worry that a supply-only ventilation system (for example, central-fan-integrated supply ventilation) won’t work in a cold climate, because the ventilation fan will drive interior air into building cavities where moisture can condense.. This worry is needless. As energy expert Bruce Harley explains, 'The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effect. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes. … In cold climates, I believe that distributed, supply-only ventilation such as that supplied by a ducted distribution system controlled by an AirCycler, or other ducted low-flow supply ventilation, is vastly preferable to single or multi-port exhaust-only systems, except in extremely tight homes (in which case balanced supply and exhaust ventilation is the best choice).'”

    Q. "You've referenced the AirCycler control in your articles and I was wondering if you would recommended using the FanConnect option which syncs a bathroom exhaust fan with the motorized damper and the central fan call. This is supposed to provide a direct exit for exhaust air rather than pressuring it into envelope leaks."

    A. I don't recommend that approach. If you want a balanced ventilation system -- and it sounds like you do -- you should install an HRV or ERV.

    Q. "When connecting an insulated fresh air intake duct to the cold air return plenum is there any special installation methods to reduce the chance of condensation either in the fresh air intake duct or where the cold, fresh air meets the cold air return plenum metal?"

    A. Condensation doesn't happen when cold air meets cold surfaces. It happens when warm air meets cold surfaces. The problem you are worrying about doesn't occur with central-fan-integrated supply ventilation systems, because none of the ducts in your forced-air duct system will even get cold enough to become a condensing surface.

    Q. "Lastly, with a central-fan-integrated supply ventilation system how do you bring in enough make-up air in the event multiple exhaust fans were running, e.g. bathroom exhaust fan, dryer, and kitchen exhaust fan?"

    A. With a powered makeup air system. To learn when such a system is required, see this article: "Makeup Air for Range Hoods."

  113. user-7075735 | | #114

    Hi Martin,

    I've re-reviewed many of your articles and would like to confirm a few points.

    For the Q & A in the previous message:
    Q. "You've referenced the AirCycler control in your articles and I was wondering if you would recommended using the FanConnect option which syncs a bathroom exhaust fan with the motorized damper and the central fan call. This is supposed to provide a direct exit for exhaust air rather than pressuring it into envelope leaks."

    A. I don't recommend that approach. If you want a balanced ventilation system -- and it sounds like you do -- you should install an HRV or ERV.

    Why would I have to install a HRV just b/c I'm trying to avoid pressuring the majority of exhaust air into envelope leaks. The extra cost of an AirCycler control with the FanConnect option is almost nothing. I understand your not worried about condensation issues when pushing air into envelope leaks, as per your "Some builders worry...." article, but in the coldest periods of winter (I'm in a Northern climate zone) I'd rather not test that theory. If I can have the exact same central-fan-integrated supply ventilation system but with an added direct exhaust path for the majority of the exhaust air it seems like I've nearly eliminated all condensation risks (especially during the coldest periods of winter) with minimal incremental cost. There will be a small operational cost increase due to the bathroom fan running for short periods when the AirCycler schedules a fresh air call, but that can be minimized using an efficient bathroom fan motor as mentioned in many of your articles for exhaust only systems. A HRV unit is minimum $5000 installed in my area, maybe more, and the heating savings are offset by albeit smaller electrical fan costs. Assuming the HRV runs without costly repairs over it's life the payback is a long time assuming modest air exchanges per day.

    I like the central-fan-integrated supply ventilation system for all the reasons you recommend plus it's ability to maintain uniform air temperature in all the rooms in the cabin when circulating air with the fresh air damper closed. I haven't seen whether this is possible, but in addition to the AirCycler control I'd like to interlock the kitchen exhaust fan with the motorized damper and furnace fan to turn on when the kitchen exhaust fan is running in order to supply some make-up air. Not sure if the AirCycler and kitchen exhaust fan can be connected to the same motorized damper and function properly. For my situation I don't need a large, heated make-up air system and would prefer to control where the make-up air is sourced from i.e. come in the fresh air intake duct, go through the furnace filter, and then be distributed by the duct system vs. pulling air through unknown envelope leaks.

    Always appreciate discussing these issues with you Martin.
    -Arden

  114. User avater GBA Editor
    Martin Holladay | | #115

    Arden,
    For homeowners like you who want a balanced ventilation system, I recommend an HRV or an ERV, because these systems are easier to balance and provide heat recovery.

    If instead, you want to install a central-fan-integrated supply ventilation system coupled with an AirCycler that controls your bathroom exhaust fans, go ahead -- it's your house.

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